Compact mode-locked laser module

ABSTRACT

Apparatus and methods for producing ultrashort optical pulses are described. A high-power, solid-state, passively mode-locked laser can be manufactured in a compact module that can be incorporated into a portable instrument. The mode-locked laser can produce sub-50-ps optical pulses at a repetition rates between 200 MHz and 50 MHz, rates suitable for massively parallel data-acquisition. The optical pulses can be used to generate a reference clock signal for synchronizing data-acquisition and signal-processing electronics of the portable instrument.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application No.62/435,688, filed Dec. 16, 2016 and titled “Compact Mode-Locked LaserModule,” which is incorporated by reference in its entirety.

FIELD

The present application is directed to compact apparatus and methods forproducing sub-100-picosecond optical pulses. The apparatus may beincorporated into instrumentation that uses the optical pulses foranalytical, medical, manufacturing, or communication purposes.

BACKGROUND

Ultrashort optical pulses (i.e., optical pulses less than about 100picoseconds) are useful in various areas of research and development aswell as commercial applications. For example, ultrashort optical pulsesmay be useful for time-domain spectroscopy, optical ranging, time-domainimaging (TDI), optical coherence tomography (OCT), fluorescent lifetimeimaging (FLI), and lifetime-resolved fluorescent detection for geneticsequencing. Ultrashort pulses may also be useful for commercialapplications including optical communication systems, medicalapplications, and testing of optoelectronic devices.

Conventional mode-locked lasers have been developed to produceultrashort optical pulses, and a variety of such lasers are currentlyavailable commercially. For example, some solid-state lasers and fiberlasers have been developed to deliver pulses with durations well below200 femtoseconds. However, for some applications, these pulse durationsmay be shorter than is needed and the cost of these lasing systems maybe prohibitively high for certain applications. Additionally, theselasing systems may be stand-alone systems that have a sizeable footprint(e.g., on the order of 1 ft² or larger), have appreciable weight, andoccupy a sizeable volume (e.g., 0.5 ft³ or larger). Such lasing systemsare not readily portable or incorporated into other portable systems asa module.

SUMMARY

The technology described herein relates to apparatus and methods forproducing ultrashort optical pulses. A mode-locked laser system isdescribed that can be implemented as a compact, low-cost laser modulethat is capable of producing sub-100-picosecond pulses atpulse-repetition rates as low as 50 MHz. The optical pulses from thelaser can be detected electronically with circuitry included in themodule, and the resulting signal can be processed to produce anelectronic clock signal that can be used to synchronize other electronicsystems with the stream of pulses (e.g., synchronize data-acquisitionelectronics of an instrument into which the laser module isincorporated). The inventors have recognized and appreciated that acompact, low-cost, pulsed-laser system can be incorporated intoinstrumentation (e.g., time-of-flight imaging instruments, bioanalyticalinstruments that utilize lifetime-resolved fluorescent detection,genetic sequencing instruments, optical coherence tomographyinstruments, etc.), and can allow such instrumentation to become readilyportable and produced at appreciably lower cost than is the case forconventional instrumentation requiring an ultrashort pulsed laser. Highportability can make such instruments more useful for research,development, clinical use, field deployment, and commercialapplications. In an example application, the compact laser module can beincorporated into a portable genetic sequencing instrument, and theoptical pulses can be delivered to reaction chambers wheresingle-molecule sequencing events are detected.

Some embodiments relate to a mode-locked laser module comprising a basechassis; a mode-locked laser having a laser cavity assembled on the basechassis; and a gain medium located in the laser cavity that exhibits athermal lensing value between four diopters and 15 diopters when themode-locked laser is producing optical pulses.

Some embodiments relate to a mode-locked laser module comprising a basechassis; a mode-locked laser having a laser cavity assembled on the basechassis; an output coupler mounted on a first mount at a first end ofthe laser cavity, wherein the first mount provides no angular adjustmentof the output coupler with respect to an optical axis of an intracavitybeam that is incident on the output coupler; a saturable absorber mirrormounted on a second mount at a second end of the laser cavity, whereinthe second mount provides no angular adjustment of the saturableabsorber mirror with respect to the optical axis of the intracavity beamthat is incident on the saturable absorber mirror; and a gain mediumlocated between the mode-locked laser and the output coupler.

Some embodiments relate to a mode-locked laser module comprising a basechassis; an output coupler and a first focusing optic mounted on thebase chassis; a saturable absorber mirror and second focusing opticmounted on the base chassis, wherein the output coupler and saturableabsorber mirror comprise end mirrors of a laser cavity for themode-locked laser; a gain medium located along an optical axis of anintracavity beam within the laser cavity; and a cavity length extendingregion comprising two reflectors located between the output coupler andthe saturable absorber mirror, wherein the two reflectors fold theintracavity beam more than two times.

Some embodiments relate to a mode-locked laser module comprising a basechassis; a mode-locked laser having a first laser cavity configured tooperate at a pulse repetition rate between 50 MHz and 200 MHz, whereinthe mode-locked laser is assembled on the base chassis; a first endmirror of the first laser cavity located at a first end of the firstlaser cavity; a second end mirror of the first laser cavity located at asecond end of the first laser cavity; and a gain medium located withinthe first laser cavity, wherein the gain medium is configured to exhibitthermal lensing when pumped at an operating power for the first lasercavity, wherein the thermal lensing supports lasing in a second lasercavity formed within the first laser cavity that is less than one-halfthe length of the first laser cavity and that includes the first endmirror and a third end mirror that is installed on the base chassis inthe first laser cavity.

Some embodiments relate to a method of operating a mode-locked laser,the method comprising pumping a gain medium of a laser cavity with anoptical pump beam, such that the gain medium exhibits thermal lensinghaving a range of diopter values between 8 diopters and 12 diopters;reflecting an intracavity beam from and output coupler at a first end ofthe laser cavity and a saturable absorber mirror at a second end of thelaser cavity; and producing an output of stable optical pulses over therange of diopter values.

The foregoing and other aspects, implementations, acts, functionalities,features and, embodiments of the present teachings can be more fullyunderstood from the following description in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the figures, described herein,are for illustration purposes only. It is to be understood that in someinstances various aspects of the invention may be shown exaggerated orenlarged to facilitate an understanding of the invention. In thedrawings, like reference characters generally refer to like features,functionally similar and/or structurally similar elements throughout thevarious figures. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the teachings.The drawings are not intended to limit the scope of the presentteachings in any way.

FIG. 1-1A is a block diagram depiction of an analytical instrument thatincludes a compact mode-locked laser module, according to someembodiments.

FIG. 1-1B depicts a compact mode-locked laser incorporated into ananalytical instrument, according to some embodiments.

FIG. 1-2 depicts a train of optical pulses, according to someembodiments.

FIG. 1-3 depicts an example of parallel reaction chambers that can beexcited optically by a pulsed laser via one or more waveguides andcorresponding detectors for each chamber, according to some embodiments.

FIG. 1-4 illustrates optical excitation of a reaction chamber from awaveguide, according to some embodiments.

FIG. 1-5 depicts further details of an integrated reaction chamber,optical waveguide, and time-binning photodetector, according to someembodiments.

FIG. 1-6 depicts an example of a biological reaction that can occurwithin a reaction chamber, according to some embodiments.

FIG. 1-7 depicts emission probability curves for two differentfluorophores having different decay characteristics.

FIG. 1-8 depicts time-binning detection of fluorescent emission,according to some embodiments.

FIG. 1-9 depicts a time-binning photodetector, according to someembodiments.

FIG. 1-10A depicts pulsed excitation and time-binned detection offluorescent emission from a sample, according to some embodiments.

FIG. 1-10B depicts a histogram of accumulated fluorescent photon countsin various time bins after repeated pulsed excitation of a sample,according to some embodiments.

FIG. 1-11A-1-11D depict different histograms that may correspond to thefour nucleotides (T, A, C, G) or nucleotide analogs, according to someembodiments.

FIG. 2-1 depicts a compact mode-locked laser module, according to someembodiments.

FIG. 3-1A illustrates a mount for a gain medium or other high-poweroptical component which can be used in a compact mode-locked laser,according to some embodiments.

FIG. 3-1B illustrates a mount for a gain medium or other high-poweroptical component which can be used in a compact mode-locked laser,according to some embodiments.

FIG. 3-2A depicts an integrated optical mount, according to someembodiments.

FIG. 3-2B depicts an optic mounted in an integrated optical mount,according to some embodiments.

FIG. 3-3 depicts a saturable-absorber mirror and mount, according tosome implementations.

FIG. 3-4 depicts an integrated optical mount, according to someembodiments.

FIG. 3-5A through FIG. 3-5D depict various embodiments ofoptical-path-length extenders which can be incorporated as part of alaser cavity, according to some implementations.

FIG. 3-6A depicts, in plan view, a platform for mounting a gain mediumor other high-power optical system which can be used in a compactmode-locked laser, according to some embodiments.

FIG. 3-6B and FIG. 3-6C depict elevation views of the platformillustrated in FIG. 3-6A, according to some embodiments.

FIG. 4-1 depicts a diode-laser pump module, according to someembodiments.

FIG. 4-2A depicts an elevation view of an example adjustable, kinematicmounting assembly;

FIG. 4-2B depicts a plan view of an example adjustable, kinematicmounting assembly;

FIG. 5-1 depicts a system for synchronizing instrument electronics totiming of optical pulses, according to some embodiments.

FIG. 5-2 depicts clock-generation circuitry for an analytical instrumentthat incorporates a pulsed optical source, according to someembodiments.

FIG. 5-3 depicts system circuitry, according to some embodiments.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings. When describing embodiments in referenceto the drawings, directional references (“above,” “below,” “top,”“bottom,” “left,” “right,” “horizontal,” “vertical,” etc.) may be used.Such references are intended merely as an aid to the reader viewing thedrawings in a normal orientation. These directional references are notintended to describe a preferred or only orientation of features of anembodied device. A device may be embodied using other orientations.

DETAILED DESCRIPTION I. Introduction

The inventors have recognized and appreciated that conventionalultrashort-pulsed lasers capable of providing average output powers ofat least 500 mW are typically large, expensive, and unsuitable for manymobile applications. Such lasers are typically too large and heavy toincorporate into portable instrumentation that can be adapted forimaging, ranging, or table-top bioanalytical applications. Accordingly,the inventors have conceived of compact, ultrashort-pulsed lasingsystems that can provide sub-100-picosecond pulses at selectedwavelengths and at average optical powers as high as 3.5 Watts (W). Thelasing system can be configured to provide a repetition rate of opticalpulses between about 50 MHz and about 200 MHz, which is well suited formassively parallel data acquisition. In some embodiments, an areaoccupied by a mode-locked laser module and its optics can be about thesize of an A4 sheet of paper with a thickness of about 40 mm or less. Avolume occupied by the module may be at most 0.07 ft³, which is nearly afactor of 10 reduction in volume occupied by conventionalultrashort-pulsed lasers that cannot deliver as much optical power.Because the laser has a compact slab form factor, it can be readilyincorporated into an instrument as a replaceable module, e.g., a moduleto swap in or out as one might add or exchange boards on a personalcomputer.

The term “optical” may refer to ultra-violet, visible, near-infrared,and short-wavelength infrared spectral bands.

In the area of bioanalytical technologies, such a compact mode-lockedlaser module can be used to deliver optical excitation energy to aplurality of reaction chambers integrated onto a chip, for example. Thenumber of reaction chambers on the chip can be between about 10,000 andabout 10,000,000, and the chambers can contain samples that can undergomultiple biochemical reactions over a period of time, according to someimplementations. In other implementations, there can be fewer or morereaction chambers on the chip. According to some embodiments, thesamples or molecules interacting with the samples can be labeled withone or more fluorophores that fluoresce(s), or the samples may fluorescethemselves, following excitation by an optical pulse from themode-locked laser module. Detection and analysis of fluorescence fromthe reaction chambers provides information about the samples within thechambers.

To make a portable instrument that includes such a large number ofreaction chambers and that uses multiple different fluorophores,requires addressing several technical challenges. A pulsed lasing systemmust be small and lightweight, and it must provide enough optical power(e.g., more than about 300 mW at a suitable excitation wavelength) toexcite fluorophores in all the reaction chambers. The pulsed lasingsystem may also be required to produce a stream of ultrashort opticalpulses that is stable over the duration of an assay or sequencing run,which can last for tens of minutes or hours. Additionally, there must besome way to excite different fluorophores with the mode-locked laser(e.g., four fluorophores with different emission characteristics for DNAsequencing), and detect different emission characteristics atappropriate times at each reaction chamber from the fluorophores so thateach fluorophore can be distinguished from the other fluorophores toobtain useful information. Further, for applications involvingintegrated optical circuits on a chip, there must be some way to adaptan output beam from the laser module to match receiving optics at thechip, and to maintain stable and efficient coupling to the chip overlong periods of time.

A compact mode-locked laser according to the present embodiments can beincorporated as an interchangeable module into portable instruments. Aform factor for the module is a slab shape measuring no greater than 350mm on a longest edge of the slab and having a thickness no greater than40 mm, occupying a volume no greater than 0.1 cubic foot. Inembodiments, a longest edge dimension can be a value between 300 mm and350 mm, and a largest thickness can be a value between 30 mm and 40 mm.The weight of the module can be no greater than 2 kilograms, andoperating power consumed by the module can be no more than 20 Watts. Inembodiments, a maximum weight can be a value between 1 kilogram and 20kilograms, and a maximum operating power can be a value between 10 Wattsand 20 Watts. The laser can produce a stable train of sub-40-picosecondpulses at an excitation wavelength of approximately 532 nm at averageoutput powers that can be controlled between 100 mW and 1.5 W.Mode-locked operation at a selected output power can be stable forhours. The module also includes circuitry for sensing optical pulses andoptical power levels produced by the laser. A signal derived fromsensing the optical pulses can be used to generate an electronic clocksignal that can be used to synchronize instrument electronics (e.g.,data acquisition cycles) with the timing of optical pulses produced bythe laser.

II. Example Bioanalytical Application

By way of explanation, a bioanalytical application is described in whicha compact mode-locked laser module is used to excite fluorophores in aplurality of reaction chambers on a chip. The example application isintended to highlight some of the more demanding requirements for thelaser module, and is not intended to limit the laser module to onlybioanalytic applications. The module can be used for other technologiessuch as communications, imaging, photonic chip or electronic chipprobing and diagnosis, manufacturing (cutting, ablating), and medicaltreatment and diagnosis.

In overview, a portable analytic instrument 1-100 may comprise one ormore mode-locked laser modules 1-108 mounted as a replaceable modulewithin, or otherwise coupled to, the instrument, as depicted in FIG.1-1A. The portable analytic instrument 1-100 can include an opticalsystem 1-115 and an analytic system 1-160. The optical system 1-115 caninclude some combination of optical components (which may include, forexample, none, one, or more of each of: lens, mirror, optical filter,attenuator, beam-steering component, beam shaping component) and beconfigured to operate on and/or deliver output optical pulses 1-122 froma mode-locked laser module 1-108 to the analytic system 1-160. Theanalytic system can include a plurality of components that are arrangedto direct the optical pulses to at least one sample that is to beanalyzed, receive one or more optical signals (e.g., fluorescence,backscattered radiation) from the at least one sample, and produce oneor more electrical signals representative of the received opticalsignals. In some embodiments, the analytic system 1-160 can include oneor more photodetectors and signal-processing electronics (e.g., one ormore microcontrollers, one or more field-programmable gate arrays, oneor more microprocessors, one or more digital signal processors, logicgates, etc.) configured to process the electrical signals from thephotodetectors. The analytic system 1-160 can also include datatransmission hardware configured to transmit and receive data to andfrom external devices via one or more data communications links. In someembodiments, the analytic system 1-160 can be configured to receive abio-optoelectronic chip 1-140, which holds one or more samples to beanalyzed.

Although the optical pulses 1-122 are depicted in the drawing as havinga single transverse optical mode, in some embodiments, the opticaloutput from a mode-locked laser module 1-108 may be multimodal (e.g.,have a higher-order transverse mode). In such embodiments, a transverseoutput beam profile may have multiple intensity peaks and minima due tomultimodal operation of the laser. In some embodiments, a multimodaloutput can be homogenized (e.g., by diffusing optics) by the opticalsystem 1-115 or when coupled to the analytic system 1-160. In someimplementations, a multimodal output can be coupled to a plurality offibers or waveguides in the analytic system 1-160. For example, eachintensity peak of a multimodal output can be coupled to a separatewaveguide or waveguides in an array of waveguides that connect to thebio-optoelectronic chip 1-140. Allowing a mode-locked laser to operatein a multimode state can enable higher output powers from themode-locked laser.

FIG. 1-1B depicts a further detailed example of a portable analyticalinstrument 1-100 that includes a compact mode-locked laser module 1-108.In some implementations, the module can be mounted to an instrumentchassis or frame 1-102 of the instrument, and may be located inside anouter casing of the instrument. According to some embodiments, amode-locked laser module 1-108 can include a mode-locked laser 1-110 andadditional components that can be used to operate the mode-locked laserand operate on an output beam from the mode-locked laser. Themode-locked laser 1-110 may comprise an element (e.g., saturableabsorber, acoustooptic modulator, Kerr lens) in a laser cavity, orcoupled to the laser cavity, that induces phase locking of the laser'slongitudinal frequency modes. The laser cavity can be defined in part bycavity end mirrors 1-111, 1-119. In some implementations, a mode-lockedlaser 1-110 can be passively mode locked, e.g., by a saturable absorber.Such locking of the frequency modes results in pulsed operation of thelaser (e.g., an intracavity pulse 1-120 bounces back-and-forth betweenthe cavity end mirrors) and produces a stream of output optical pulses1-122 from one end mirror 1-111 which is partially transmitting.

In some cases, the analytic instrument 1-100 can be configured toreceive a removable, packaged, bio-optoelectronic chip 1-140. The chipcan include a plurality of reaction chambers, integrated opticalcomponents arranged to deliver optical excitation energy to the reactionchambers, and integrated photodetectors arranged to detect fluorescentemission from the reaction chambers. In some implementations, the chip1-140 can be disposable, whereas in other implementations the chip canbe reusable. When the chip is received by the instrument, it can be inelectrical and optical communication with the mode-locked laser andelectrical and optical communication with the analytic system 1-160.

In some embodiments, the bio-optoelectronic chip can be mounted (e.g.,via a socket connection) on an electronic circuit board 1-130, such as aprinted circuit board (PCB) that can include additional instrumentelectronics. For example, the PCB 1-130 can include circuitry configuredto provide electrical power, one or more clock signals, and controlsignals to the bio-optoelectronic chip 1-140, and signal-processingcircuitry arranged to receive signals representative of fluorescentemission detected from the reaction chambers. Data returned from thebio-optoelectronic chip can be processed in part or entirely by theinstrument, although data may be transmitted via a network connection toone or more remote data processors, in some implementations. The PCB1-130 can also include circuitry configured to receive feedback signalsfrom the chip relating to optical coupling and power levels of theoptical pulses 1-122 coupled into waveguides of the bio-optoelectronicchip 1-140. The feedback signals may be provided to one or both of thelaser module 1-108 and optical system 1-115 to control one or moreparameters of the output beam of optical pulses 1-122. In some cases,the PCB 1-130 can provide or route power to the laser module 1-108 foroperating the mode-locked laser and circuitry in the laser module.

According to some embodiments, a mode-locked laser 1-110 can comprise again medium 1-105 (which can be solid-state material in someembodiments), an output coupler 1-111, and a laser-cavity end mirror1-119. The mode-locked laser's optical cavity can be bound by the outputcoupler 1-111 and end mirror 1-119. An optical axis 1-125 of the lasercavity can have one or more folds (turns) to increase the length of thelaser cavity. In some embodiments, there can be additional opticalelements (not shown in FIG. 1-1B) in the laser cavity for beam shaping,wavelength selection, and/or pulse forming. In some cases, the endmirror 1-119 comprises a saturable-absorber mirror (SAM) that inducespassive mode locking of longitudinal cavity modes and results in pulsedoperation of the laser 1-110. The laser module 1-108 can further includea pump source (e.g., a laser diode, not shown in FIG. 1-1B) for excitingthe gain medium.

When the laser 1-110 is mode locked, an intracavity pulse 1-120 cancirculate between the end mirror 1-119 and the output coupler 1-111, anda portion of the intracavity pulse can be transmitted through the outputcoupler 1-111 as an output pulse 1-122. Accordingly, a train of outputpulses 1-122, as depicted in the graph of FIG. 1-2, can be detected atthe output coupler as the intracavity pulse 1-120 bounces back-and-forthbetween the output coupler 1-111 and end mirror 1-119 in the lasercavity.

FIG. 1-2 depicts temporal intensity profiles of the output pulses 1-122.In some embodiments, the peak intensity values of the emitted pulses maybe approximately equal, and the profiles may have a Gaussian temporalprofile, though other profiles such as a sech² profile may be possible.In some cases, the pulses may not have symmetric temporal profiles andmay have other temporal shapes. The duration of each pulse may becharacterized by a full-width-half-maximum (FWHM) value, as indicated inFIG. 1-2. According to some embodiments of a mode-locked laser,ultrashort optical pulses can have FWHM values less than 100 picoseconds(ps). In some cases, the FWHM values can be between approximately 5 psand approximately 30 ps.

The output pulses 1-122 can be separated by regular intervals T. Forexample, T can be determined by a round-trip travel time between theoutput coupler 1-111 and cavity end mirror 1-119. According to someembodiments, the pulse-separation interval T can be between about 1 nsand about 30 ns. In some cases, the pulse-separation interval T can bebetween about 5 ns and about 20 ns, corresponding to a laser-cavitylength (an approximate length of the optical axis 1-125 within the lasercavity) between about 0.7 meter and about 3 meters. In embodiments, thepulse-separation interval corresponds to a round trip travel time in thelaser cavity, so that a cavity length of 3 meters (round-trip distanceof 6 meters) provides a pulse-separation interval T of approximately 20ns.

According to some embodiments, a desired pulse-separation interval T andlaser-cavity length can be determined by a combination of the number ofreaction chambers on the chip 1-140, fluorescent emissioncharacteristics, and the speed of data-handling circuitry for readingdata from the bio-optoelectronic chip 1-140. The inventors haverecognized and appreciated that different fluorophores can bedistinguished by their different fluorescent decay rates orcharacteristic lifetimes. Accordingly, there needs to be a sufficientpulse-separation interval T to collect adequate statistics for theselected fluorophores to distinguish between their different decayrates. Additionally, if the pulse-separation interval T is too short,the data handling circuitry cannot keep up with the large amount of databeing collected by the large number of reaction chambers. The inventorshave recognized and appreciated that a pulse-separation interval Tbetween about 5 ns and about 20 ns is suitable for fluorophores thathave decay rates up to about 2 ns and for handling data from betweenabout 60,000 and 8,000,000 reaction chambers.

According to some implementations, a beam-steering module 1-150 canreceive output pulses from the mode-locked laser module 1-108 and beconfigured to adjust at least the position and incident angles of theoptical pulses onto an optical coupler of the bio-optoelectronic chip1-140. In some cases, the output pulses from the mode-locked lasermodule can be operated on by a beam-steering module to additionally oralternatively change a beam shape and/or beam rotation at an opticalcoupler on the bio-optoelectronic chip 1-140. In some implementations,the beam-steering module 1-150 can further provide focusing and/orpolarization adjustments of the beam of output pulses onto the opticalcoupler. One example of a beam-steering module is described in U.S.patent application Ser. No. 15/161,088 titled “Pulsed Laser andBioanalytic System,” filed May 20, 2016, which is incorporated herein byreference. Another example of a beam-steering module is described in aseparate U.S. patent application No. 62,435,679, filed Dec. 16, 2016 andtitled “Compact Beam Shaping and Steering Assembly,” which isincorporated herein by reference.

Referring to FIG. 1-3, the output pulses 1-122 from a mode-locked lasermodule can be coupled into one or more optical waveguides 1-312 on thebio-optoelectronic chip. In some embodiments, the optical pulses can becoupled to one or more waveguides via a grating coupler 1-310, thoughcoupling to an end of one or more optical waveguides on thebio-optoelectronic chip can be used in some embodiments. According tosome embodiments, a quad detector 1-320 can be located on asemiconductor substrate 1-305 (e.g., a silicon substrate) for aiding inalignment of the beam of optical pulses 1-122 to a grating coupler1-310. The one or more waveguides 1-312 and reaction chambers 1-330 canbe integrated on the same semiconductor substrate with interveningdielectric layers (e.g., silicon dioxide layers) between the substrate,waveguide, reaction chambers, and photodetectors 1-322.

Each waveguide 1-312 can include a tapered portion 1-315 below thereaction chambers 1-330 to equalize optical power coupled to thereaction chambers along the waveguide. The reducing taper can force moreoptical energy outside the waveguide's core, increasing coupling to thereaction chambers and compensating for optical losses along thewaveguide, including losses for light coupling into the reactionchambers. A second grating coupler 1-317 can be located at an end ofeach waveguide to direct optical energy to an integrated photodiode1-324. The integrated photodiode can detect an amount of power coupleddown a waveguide and provide a detected signal to feedback circuitrythat controls the beam-steering module 1-150, for example.

The reaction chambers 1-330 can be aligned with the tapered portion1-315 of the waveguide and recessed in a tub 1-340. There can betime-binning photodetectors 1-322 located on the semiconductor substrate1-305 for each reaction chamber 1-330. A metal coating and/or multilayercoating 1-350 can be formed around the reaction chambers and above thewaveguide to prevent optical excitation of fluorophores that are not inthe reaction chambers (e.g., dispersed in a solution above the reactionchambers). The metal coating and/or multilayer coating 1-350 may beraised beyond edges of the tub 1-340 to reduce absorptive losses of theoptical energy in the waveguide 1-312 at the input and output ends ofeach waveguide.

There can be a plurality of rows of waveguides, reaction chambers, andtime-binning photodetectors on the bio-optoelectronic chip 1-140. Forexample, there can be 128 rows, each having 512 reaction chambers, for atotal of 65,536 reaction chambers in some implementations. Otherimplementations may include fewer or more reaction chambers, and mayinclude other layout configurations. Optical power from the mode-lockedlaser 1-110 can be distributed to the multiple waveguides via one ormore star couplers or multi-mode interference couplers, or by any othermeans, located between an optical coupler to the chip 1-140 and theplurality of waveguides.

FIG. 1-4 illustrates optical energy coupling from an optical pulse 1-122within a waveguide 1-315 to a reaction chamber 1-330. The drawing hasbeen produced from an electromagnetic field simulation of the opticalwave that accounts for waveguide dimensions, reaction chamberdimensions, the different materials' optical properties, and thedistance of the waveguide 1-315 from the reaction chamber 1-330. Thewaveguide can be formed from silicon nitride in a surrounding medium1-410 of silicon dioxide, for example. The waveguide, surroundingmedium, and reaction chamber can be formed by microfabrication processesdescribed in U.S. application Ser. No. 14/821,688, filed Aug. 7, 2015,titled “Integrated Device for Probing, Detecting and AnalyzingMolecules”. According to some embodiments, an evanescent optical field1-420 couples optical energy transported by the waveguide to thereaction chamber 1-330.

A non-limiting example of a biological reaction taking place in areaction chamber 1-330 is depicted in FIG. 1-5. In this example,sequential incorporation of nucleotides or nucleotide analogs into agrowing strand that is complementary to a target nucleic acid is takingplace in the reaction chamber. The sequential incorporation can bedetected to sequence DNA. The reaction chamber can have a depth betweenabout 150 nm and about 250 nm and a diameter between about 80 nm andabout 160 nm. A metallization layer 1-540 (e.g., a metallization for anelectrical reference potential) can be patterned above the photodetectorto provide an aperture that blocks stray light from adjacent reactionchambers and other unwanted light sources. According to someembodiments, polymerase 1-520 can be located within the reaction chamber1-330 (e.g., attached to a base of the chamber). The polymerase can takeup a target nucleic acid 1-510 (e.g., a portion of nucleic acid derivedfrom DNA), and sequence a growing strand of complementary nucleic acidto produce a growing strand of DNA 1-512. Nucleotides or nucleotideanalogs labeled with different fluorophores can be dispersed in asolution above and within the reaction chamber.

When a labeled nucleotide or nucleotide analog 1-610 is incorporatedinto a growing strand of complementary nucleic acid, as depicted in FIG.1-6, one or more attached fluorophores 1-630 can be repeatedly excitedby pulses of optical energy coupled into the reaction chamber 1-330 fromthe waveguide 1-315. In some embodiments, the fluorophore orfluorophores 1-630 can be attached to one or more nucleotides ornucleotide analogs 1-610 with any suitable linker 1-620. Anincorporation event may last for a period of time up to about 100 ms.During this time, pulses of fluorescent emission resulting fromexcitation of the fluorophore(s) by pulses from the mode-locked lasercan be detected with a time-binning photodetector 1-322. In someembodiments, there can be one or more additional integrated devices1-323 at each pixel for signal handling (e.g., amplification, read-out,routing, etc.). According to some embodiments, each pixel can include asingle or multilayer optical filter 1-530 that passes fluorescentemission and reduces transmission of radiation from the excitationpulse. Some implementations may not use the optical filter 1-530. Byattaching fluorophores with different emission characteristics (e.g.,fluorescent decay rates, intensity, fluorescent wavelength) to thedifferent nucleotides (A, C, G, T), detecting and distinguishing thedifferent emission characteristics while the strand of DNA 1-512incorporates a nucleic acid and enables determination of the geneticsequence of the growing strand of DNA.

According to some embodiments, an analytical instrument 1-100 that isconfigured to analyze samples based on fluorescent emissioncharacteristics can detect differences in fluorescent lifetimes and/orintensities between different fluorescent molecules, and/or differencesbetween lifetimes and/or intensities of the same fluorescent moleculesin different environments. By way of explanation, FIG. 1-7 plots twodifferent fluorescent emission probability curves (A and B), which canbe representative of fluorescent emission from two different fluorescentmolecules, for example. With reference to curve A (dashed line), afterbeing excited by a short or ultrashort optical pulse, a probabilityp_(A)(t) of a fluorescent emission from a first molecule may decay withtime, as depicted. In some cases, the decrease in the probability of aphoton being emitted over time can be represented by an exponentialdecay function p_(A)(t)=P_(A0)e^(−t/τ) ^(A) , where P_(A0) is an initialemission probability and τ_(A) is a temporal parameter associated withthe first fluorescent molecule that characterizes the emission decayprobability. τ_(A) may be referred to as the “fluorescence lifetime,”“emission lifetime,” or “lifetime” of the first fluorescent molecule. Insome cases, the value of TA can be altered by a local environment of thefluorescent molecule. Other fluorescent molecules can have differentemission characteristics than that shown in curve A. For example,another fluorescent molecule can have a decay profile that differs froma single exponential decay, and its lifetime can be characterized by ahalf-life value or some other metric.

A second fluorescent molecule may have a decay profile that isexponential, but has a measurably different lifetime τ_(B), as depictedfor curve B in FIG. 1-7. In the example shown, the lifetime for thesecond fluorescent molecule of curve B is shorter than the lifetime forcurve A, and the probability of emission is higher sooner afterexcitation of the second molecule than for curve A. Differentfluorescent molecules can have lifetimes or half-life values rangingfrom about 0.1 ns to about 20 ns, in some embodiments.

The inventors have recognized and appreciated that differences influorescent emission lifetimes can be used to discern between thepresence or absence of different fluorescent molecules and/or to discernbetween different environments or conditions to which a fluorescentmolecule is subjected. In some cases, discerning fluorescent moleculesbased on lifetime (rather than emission wavelength, for example) cansimplify aspects of an analytical instrument 1-100. As an example,wavelength-discriminating optics (such as wavelength filters, dedicateddetectors for each wavelength, dedicated pulsed optical sources atdifferent wavelengths, and/or diffractive optics) can be reduced innumber or eliminated when discerning fluorescent molecules based onlifetime. In some cases, a single pulsed optical source operating at asingle characteristic wavelength can be used to excite differentfluorescent molecules that emit within a same wavelength region of theoptical spectrum but have measurably different lifetimes. An analyticsystem that uses a single pulsed optical source, rather than multiplesources operating at different wavelengths, to excite and discerndifferent fluorescent molecules emitting in a same wavelength region canbe less complex to operate and maintain, more compact, and can bemanufactured at lower cost.

Although analytic systems based on fluorescent lifetime analysis canhave certain benefits, the amount of information obtained by an analyticsystem and/or detection accuracy can be increased by allowing foradditional detection techniques. For example, some analytic systems1-160 can additionally be configured to discern one or more propertiesof a sample based on fluorescent wavelength and/or fluorescentintensity.

Referring again to FIG. 1-7, according to some embodiments, differentfluorescent lifetimes can be distinguished with a photodetector that isconfigured to time-bin fluorescent emission events following excitationof a fluorescent molecule. The time binning can occur during a singlecharge-accumulation cycle for the photodetector. A charge-accumulationcycle is an interval between read-out events during whichphoto-generated carriers are accumulated in bins of the time-binningphotodetector. The concept of determining fluorescent lifetime bytime-binning of emission events is introduced graphically in FIG. 1-8.At time t₀ just prior to t₁, a fluorescent molecule or ensemble offluorescent molecules of a same type (e.g., the type corresponding tocurve B of FIG. 1-7) is (are) excited by a short or ultrashort opticalpulse. For a large ensemble of molecules, the intensity of emission canhave a time profile similar to curve B, as depicted in FIG. 1-8.

For a single molecule or a small number of molecules, however, theemission of fluorescent photons occurs according to the statistics ofcurve B in FIG. 1-7, for this example. A time-binning photodetector1-322 can accumulate carriers generated from emission events intodiscrete time bins (three indicated in FIG. 1-8) that are temporallyresolved with respect to the excitation time of the fluorescentmolecule(s). When a large number of emission events are summed, carriersaccumulated in the time bins can approximate the decaying intensitycurve shown in FIG. 1-8, and the binned signals can be used todistinguish between different fluorescent molecules or differentenvironments in which a fluorescent molecule is located.

Examples of a time-binning photodetector 1-322 are described in U.S.patent application Ser. No. 14/821,656, filed Aug. 7, 2015, titled“Integrated Device for Temporal Binning of Received Photons,” which isincorporated herein by reference. For explanation purposes, anon-limiting embodiment of a time-binning photodetector is depicted inFIG. 1-9. A single time-binning photodetector 1-900 can comprise aphoton-absorption/carrier-generation region 1-902, a carrier-travelregion 1-906, and a plurality of carrier-storage bins 1-908 a, 1-908 b,1-908 c all formed on a semiconductor substrate. The carrier-travelregion can be connected to the plurality of carrier-storage bins bycarrier-transport channels 1-907. Only three carrier-storage bins areshown, but there may be more. There can be a read-out channel 1-910connected to the carrier-storage bins. Thephoton-absorption/carrier-generation region 1-902, carrier-travel region1-906, carrier-storage bins 1-908 a, 1-908 b, 1-908 c, and read-outchannel 1-910 can be formed by doping the semiconductor locally and/orforming adjacent insulating regions to provide photodetection capabilityand confine carriers. A time-binning photodetector 1-900 can alsoinclude a plurality of electrodes 1-920, 1-922, 1-932, 1-934, 1-936,1-940 formed on the substrate that are configured to generate electricfields in the device for transporting carriers through the device.

In operation, fluorescent photons may be received at thephoton-absorption/carrier-generation region 1-902 at different times andgenerate carriers. For example, at approximately time t₁ threefluorescent photons may generate three carrier electrons in a depletionregion of the photon-absorption/carrier-generation region 1-902. Anelectric field in the device (due to doping and/or an externally appliedbias to electrodes 1-920 and 1-922, and optionally or alternatively to1-932, 1-934, 1-936) can move the carriers to the carrier-travel region1-906. In the carrier-travel region, distance of travel translates to atime after excitation of the fluorescent molecules. At a later time t₅,another fluorescent photon may be received in thephoton-absorption/carrier-generation region 1-902 and generate anadditional carrier. At this time, the first three carriers have traveledto a position in the carrier-travel region 1-906 adjacent to the secondstorage bin 1-908 b. At a later time t₇, an electrical bias can beapplied between electrodes 1-932, 1-934, 1-936 and electrode 1-940 tolaterally transport carriers from the carrier-travel region 1-906 to thestorage bins. The first three carriers can then be transported to andretained in the first bin 1-908 a and the later-generated carrier can betransported to and retained in the third bin 1-908 c. In someimplementations, the time intervals corresponding to each storage binare at the sub-nanosecond time scale, though longer time scales can beused in some embodiments (e.g., in embodiments where fluorophores havelonger decay times).

The process of generating and time-binning carriers after an excitationevent (e.g., excitation pulse from a pulsed optical source) can occuronce after a single excitation pulse or be repeated multiple times aftermultiple excitation pulses during a single charge-accumulation cycle forthe photodetector 1-900. After charge accumulation is complete, carrierscan be read out of the storage bins via the read-out channel 1-910. Forexample, an appropriate biasing sequence can be applied to at leastelectrode 1-940 and a downstream electrode (not shown) to removecarriers from the storage bins 1-908 a, 1-908 b, 1-908 c.

After a number of excitation events, the accumulated signal in eachelectron-storage bin can be read out to provide a histogram havingcorresponding bins that represent the fluorescent emission decay rate,for example. Such a process is illustrated in FIG. 1-10A and FIG. 1-10B.The histogram's bins can indicate a number of photons detected duringeach time interval after excitation of the fluorophore(s) in a reactionchamber. In some embodiments, signals for the bins will be accumulatedfollowing a large number of excitation pulses, as depicted in FIG.1-10A. The excitation pulses can occur at times t_(e1), t_(e2), t_(e3),. . . t_(eN) which are separated by the pulse interval time T. There canbe between 10⁵ and 10⁷ excitation pulses applied to the reaction chamberduring an accumulation of signals in the electron-storage bins. In someembodiments, one bin (bin 0) can be configured to detect an amplitude ofexcitation energy delivered with each optical pulse, and be used as areference signal (e.g., to normalize data).

In some implementations, only a single photon on average may be emittedfrom a fluorophore following an excitation event, as depicted in FIG.1-10A. After a first excitation event at time t_(e1), the emitted photonat time t_(f1) may occur within a first time interval, so that theresulting electron signal is accumulated in the first electron-storagebin (contributes to bin 1). In a subsequent excitation event at timet_(e2), the emitted photon at time t_(f2) may occur within a second timeinterval, so that the resulting electron signal contributes to bin 2.

After a large number of excitation events and signal accumulations, theelectron-storage bins of the time-binning photodetector 1-322 can beread out to provide a multi-valued signal (e.g., a histogram of two ormore values, an N-dimensional vector, etc.) for a reaction chamber. Thesignal values for each bin can depend upon the decay rate of thefluorophore. For example and referring again to FIG. 1-8, a fluorophorehaving a decay curve B will have a higher ratio of signal in bin 1 tobin 2 than a fluorophore having a decay curve A. The values from thebins can be analyzed and compared against calibration values, and/oreach other, to determine the particular fluorophore, which in turnidentifies the nucleotide or nucleotide analog (or any other molecule orspecimen of interest) linked to the fluorophore when in the reactionchamber.

To further aid in understanding the signal analysis, the accumulated,multi-bin values can be plotted as a histogram, as depicted in FIG.1-10B for example, or can be recorded as a vector or location inN-dimensional space. Calibration runs can be performed separately toacquire calibration values for the multi-valued signals (e.g.,calibration histograms) for four different fluorophores linked to thefour nucleotides or nucleotide analogs. As an example, the calibrationhistograms may appear as depicted in FIG. 1-11A (fluorescent labelassociated with the T nucleotide), FIG. 1-11B (fluorescent labelassociated with the A nucleotide), FIG. 1-11C (fluorescent labelassociated with the C nucleotide), and FIG. 1-11D (fluorescent labelassociated with the G nucleotide). A comparison of the measuredmulti-valued signal (corresponding to the histogram of FIG. 1-10B) tothe calibration multi-valued signals can determine the identity “T”(FIG. 1-11A) of the nucleotide or nucleotide analog being incorporatedinto the growing strand of DNA.

In some implementations, fluorescent intensity can be used additionallyor alternatively to distinguish between different fluorophores. Forexample, some fluorophores may emit at significantly differentintensities or have a significant difference in their probabilities ofexcitation (e.g., at least a difference of about 35%) even though theirdecay rates may be similar. By referencing binned signals (bins 1-3) tomeasured excitation energy bin 0, it can be possible to distinguishdifferent fluorophores based on intensity levels.

In some embodiments, different numbers of fluorophores of the same typecan be linked to different nucleotides or nucleotide analogs, so thatthe nucleotides can be identified based on fluorophore intensity. Forexample, two fluorophores can be linked to a first nucleotide (e.g.,“C”) or nucleotide analog and four or more fluorophores can be linked toa second nucleotide (e.g., “T”) or nucleotide analog. Because of thedifferent numbers of fluorophores, there may be different excitation andfluorophore emission probabilities associated with the differentnucleotides. For example, there may be more emission events for the “T”nucleotide or nucleotide analog during a signal accumulation interval,so that the apparent intensity of the bins is significantly higher thanfor the “C” nucleotide or nucleotide analog.

The inventors have recognized and appreciated that distinguishingnucleotides or any other biological or chemical specimens based onfluorophore decay rates and/or fluorophore intensities enables asimplification of the optical excitation and detection systems in ananalytical instrument 1-100. For example, optical excitation can beperformed with a single-wavelength source (e.g., a source producing onecharacteristic wavelength rather than multiple sources or a sourceoperating at multiple different characteristic wavelengths).Additionally, wavelength discriminating optics and filters may not beneeded in the detection system. Also, a single photodetector can be usedfor each reaction chamber to detect emission from differentfluorophores.

The phrase “characteristic wavelength” or “wavelength” is used to referto a central or predominant wavelength within a limited bandwidth ofradiation (e.g., a central or peak wavelength within a 20 nm bandwidthoutput by a pulsed optical source). In some cases, “characteristicwavelength” or “wavelength” may be used to refer to a peak wavelengthwithin a total bandwidth of radiation output by a source.

The inventors have recognized and appreciated that fluorophores havingemission wavelengths in a range between about 560 nm and about 900 nmcan provide adequate amounts of fluorescence to be detected by atime-binning photodetector (which can be fabricated on a silicon waferusing CMOS processes). These fluorophores can be linked to biologicalmolecules of interest such as nucleotides or nucleotide analogs.Fluorescent emission in this wavelength range can be detected withhigher responsivity in a silicon-based photodetector than fluorescenceat longer wavelengths. Additionally, fluorophores and associated linkersin this wavelength range may not interfere with incorporation of thenucleotides or nucleotide analogs into growing strands of DNA. Theinventors have also recognized and appreciated that fluorophores havingemission wavelengths in a range between about 560 nm and about 660 nmcan be optically excited with a single-wavelength source. An examplefluorophore in this range is Alexa Fluor 647, available from ThermoFisher Scientific Inc. of Waltham, Mass. The inventors have alsorecognized and appreciated that excitation energy at shorter wavelengths(e.g., between about 500 nm and about 650 nm) may be required to excitefluorophores that emit at wavelengths between about 560 nm and about 900nm. In some embodiments, the time-binning photodetectors can efficientlydetect longer-wavelength emission from the samples, e.g., byincorporating other materials, such as Ge, into the photodetectorsactive region.

Although the prospect of sequencing DNA using an excitation source thatemits a single characteristic wavelength can simplify some of theoptical system, it can place technically challenging demands on theexcitation source, as noted above. For example, the inventors haverecognized and appreciated that optical pulses from the excitationsource should extinguish quickly for the detection schemes describedabove, so that the excitation energy does not overwhelm or interferewith the subsequently detected fluorescent signal. In some embodimentsand referring again to FIG. 1-5, there may be no wavelength filtersbetween the waveguide 1-315 and the time-binning photodetector 1-322. Toavoid interference of the excitation energy with subsequent signalcollection, the excitation pulse may need to reduce in intensity by atleast 50 dB within about 100 ps from the peak of the excitation pulse.In some implementations, the excitation pulse may need to reduce inintensity by at least 80 dB within about 100 ps from the peak of theexcitation pulse. The inventors have recognized and appreciated thatmode-locked lasers can provide such rapid turn-off characteristics.However, mode-locked lasers can be difficult to operate in a stablemode-locking state for extended periods of time. Also, because the pulserepetition rate may need to be lower than 100 MHz for data acquisitionpurposes, the length of a mode-locked laser cavity can become very long.Such long lengths are contrary to a compact optical source that can beincorporated into a portable, desk-top instrument. Additionally, amode-locked laser must provide adequate energy per pulse (or highaverage powers) for excitation of fluorophores at wavelengths below 660nm, so that fluorescence is detectable with integrated photodiodes forthousands or even millions of reaction chambers in parallel. Theinventors have further recognized and appreciated that a beam quality ofthe mode-locked laser should be high (e.g., an M² value less than 1.5),so that efficient coupling can be achieved to an optical coupler andwaveguides of a bio-optoelectronic chip 1-140, for example. Currently,there is no commercial mode-locked lasing system available that providespulses at repetition rates between 50 MHz and 200 MHz, at wavelengthsbetween 500 nm and 650 nm, at average powers between 250 mW and 1 W, ina compact module (e.g., occupying a volume of less than 0.1 ft³) thatcould be incorporated into a portable, desk-top instrument and remainstable for extended periods of time.

III. Compact Mode-Locked Laser Module

The inventors have conceived and built a compact mode-locked lasermodule 1-108 (e.g., as schematically depicted in FIG. 1-1A and FIG.1-1B) that achieves the above-described performance specifications interms of average power, compactness, beam quality, pulse repetitionrate, excitation wavelength, and turn-off speed of optical pulses. Inoverview and referring to FIG. 2-1, principle components of a compactmode-locked laser module 1-108, according to some embodiments, caninclude a laser cavity (which includes optical elements between anoutput coupler 1-111 that can function as a first end mirror of thelaser cavity and saturable absorber mirror (SAM) 1-119 that can functionas a second end mirror of the laser cavity), a formed base chassis 2-105on which some or all of the components of the mode-locked laser 1-110are mounted, at least one intracavity optical element 2-128 that canstabilize operation of the mode-locked laser, frequency-doublingelements 2-170, 2-164, 2-160 that can participate in converting anoutput from the laser to a shorter wavelength, and electrical components2-190, 2-154, 2-182, 2-116 that monitor operational parameters of thelaser and generate an electronic clock signal that is synchronized tothe optical pulses produced by the laser. A pump module 2-140 can bemounted to the base chassis 2-105 and used to excite the gain medium1-105 of the mode-locked laser.

Base Chassis and Laser Cavity

The base chassis 2-105 of a compact mode-locked laser module 1-108 maymeasure between about 20 cm and about 30 cm in length L, between about10 cm and about 20 cm in height H, and has a thickness between about 10mm and about 30 mm, according to some embodiments. In some cases, one ormore of the dimensions can be up to 20% larger. According to someembodiments, a volume occupied by the compact, mode-locked laser module1-108 can be about 30 cm×18 cm×3 cm or approximately 0.07 ft³. Accordingto some implementations, the overall shape or form factor of themode-locked laser module 1-108 is a slab having a length L longer than aheight H and a thickness much less than either the length or height,occupying a volume less than 0.1 cubic foot and weighing less than, orhas a weight of, approximately, 2 kilograms. In some cases, the weightof the module 1-108 is between 1 kilogram and 2 kilograms.

In some embodiments, the base chassis 2-105 can be formed from aluminum,titanium, an alloy of aluminum, or an alloy of titanium. Other materialscan be used in other embodiments. In some implementations, the basechassis 2-105 can include a plurality of cavities 2-102 that aremachined or otherwise formed (e.g., by casting or assembly) into thebase chassis. In some embodiments, 12.5 mm-diameter optical components(or smaller) can be used to construct the mode-locked laser 1-110 and bepartially or fully recessed into the cavities 2-102 of the base chassis2-105, so that a cover (not shown) can be placed over the cavities 2-102to protect the components within the cavities from outside environmentalfactors and contaminants. In some embodiments, a cover can be placedover the cavities 2-102 to hermitically seal one or more of thecavities.

Between the cavities 2-102, there can be ribs 2-107 formed in the basechassis 2-105. In some of the ribs, there can be holes or openings (notvisible) that allow the intracavity laser beam to pass through the ribsto adjacent cavities. According to some embodiments, there can be one ormore diagonal ribs 2-107 that runs at an angle with respect to an edgeof the base chassis 2-105. For example, a diagonal rib 2-107 can run ina corner-to-corner direction across the base chassis 2-105. Theinventors have found that such a diagonal rib 2-107 can increase thetorsional stiffness of the base chassis 2-105 by a factor of threecompared to having no diagonal ribs. An increased torsional stiffnesscan help prevent instability of laser operation and improve the module'sresistance to perturbing forces acting on the base chassis 2-105. Insome cases, at least portions of the ribs can extend from a bottom of acavity to a top surface of the base chassis 2-105, so that one or morecovers (not shown) for the laser module 1-108 can attach to the ribs. Inthis regard, any suitable cover may be used including, but not limitedto, a stiff metal cover (e.g., a metal having a thickness greater thanapproximately 1 mm), a stiff polymer cover (e.g., a polymer having athickness greater than approximately 2 mm), or a flexible material(metal or polymer) that can be sealed to the base chassis 2-105, or heldagainst the base chassis 2-105 with a supporting piece (e.g., a metalframe). In some cases, a covering material comprises Tyvek®(approximately 0.25 mm thick) that is held against the base chassis witha metal frame (approximately 1.5 mm thick).

In some implementations, one or more mounting features 2-103 can belocated at one or more ribs 2-107. The mounting features 2-103 can beused to mount the compact laser module 1-108 to an instrument or otherplatform. In some cases, the mounting features provide kinematicmounting, so that each laser module 1-108, or the same laser module,reproducibly mounts in nearly the same location and alignment (e.g., towithin ±100 microns). The mounting features 2-103 may comprise holesthat are tapped or are clear. The holes can be countersunk orcounterbored. For kinematic mounting, there can be three mountingfeatures 2-103 for which the bottom surfaces (not shown in FIG. 2-1)comprise a conical contacting surface or ring contact for a firstmounting feature, a wedged contacting surface or two-point contactingsurface for a second mounting feature, and a flat surface orsingle-point contact for a third mounting feature. Alternatively, twocountersunk holes at the mounting features 2-103 may be used to alignthe base chassis 2-105 to a receiving support structure.

A mode-locked laser 1-110 of the laser module 1-108 can comprise anoutput coupler 1-111 at an output end of the laser's cavity, a gainmedium 1-105, and a saturable absorber mirror (SAM) 1-119 at an oppositeend of the laser cavity. There can be multiple mirrors 2-116, 2-117,2-120, 2-121, 2-122, 2-123, 2-124, 2-125 within the laser cavity to foldthe optical axis 1-125 of the laser and extend the length of the lasercavity to achieve a desired pulse repetition rate or pulse separationinterval T. There can also be beam-shaping optics (e.g. lenses and/orcurved mirrors) within the laser cavity to alter a size and/or shape ofthe intracavity laser beam.

Example optical components for a mode-locked laser that operates at alasing wavelength of 1064 nm will now be described. It will beappreciated that embodiments of invention are not limited to only thedescribed optical components. Fewer or more optical components may beused in some implementations (e.g., adding or removing mirrors to changethe pulse repetition rate), and the optical coatings on components canbe different for lasers that lase at different wavelengths.

The gain medium 1-105 can comprise a neodymium-doped material that ismounted in a thermally-conductive mount (e.g., an aluminum or copperblock or other thermally-conductive material) which dissipates heat intothe base chassis 2-105. The inventors have recognized that when themode-locked laser operates at high average powers (e.g., over 300 mW)thermal lensing effects in the gain medium 1-105 occur. In some cases,such thermal lensing could destabilize operation of the laser. Toimprove heat transfer from the gain medium to the thermally-conductivemount, the gain medium 1-105 can be wrapped in indium foil or any othersuitable material that improves heat transfer to thethermally-conductive mount. In some cases, silver epoxy or any othersuitable thermally-conductive adhesive can be used to secure the gaincrystal to the thermal mount. In some cases, the gain medium 1-105 andthermally-conductive mount can be mounted on a thermo-electric cooler(TEC), which can sink heat into the base chassis 2-105. The TEC or otheractive cooling techniques such as liquid cooling can provide activetemperature control of the gain medium 1-105 and reduce thermal lensingeffects.

Surprisingly, the inventors found that a laser cavity configuration, forwhich analytic modeling showed an unstable resonator, could lase stablyin the laboratory. To explain the lasing, the model had to be changed toinclude an amount of thermal lensing in the gain medium greater thanabout one diopter. According to some embodiments, a laser cavityconfiguration could be obtained in which large amounts of thermallensing could be tolerated. As a result, active cooling of the gainmedium can be removed from the mode-locked laser module 1-110. Inembodiments, the gain medium 1-105 can be disposed in athermally-conductive mount that contacts the base chassis so as toconduct heat passively from the gain medium 1-105 to the base chassis2-105.

Elimination of active cooling of the gain medium 1-105 can reduce costand complexity of the laser. The inventors have observed that activetemperature control of the gain medium need not be used for themode-locked laser 1-110 of the present embodiments, even when opticalpumping powers of up to 10 Watts are used to pump the gain medium.Surprisingly, the mode-locked laser 1-110 remains stably mode lockedover this range of pump power, even though the associated thermallensing effects (positive lensing) can change a thermally-induced focallength of the gain medium from 0 to approximately 15 diopters over thepump power range. For thermal lensing in excess of 15 diopters, thelaser cavity can become an unstable resonator that may not supportmode-locked operation nor continuous-wave operation. The stability ofmode locking over such a large range of thermal lensing in the gainmedium is due in part to the selection and arrangement of opticalcomponents for the mode-locked laser 1-110. According to one embodiment,stability and improved performance of mode-locked operation dependscritically on having an amount of thermal lensing occur in the gainmedium. In embodiments, stable mode-locked operation of the mode-lockedlaser 1-110 can be obtained for an amount of thermal lensing between onediopter and 15 diopters of positive lensing effect. Even though thethermal lensing may vary over this range, no mechanical adjustments needbe made to the laser cavity to maintain stable mode locking. Improvedperformance of the mode-locked laser can be obtained when the amount ofthermal lensing in the gain medium 1-105 is between 8 diopters and 12diopters of positive thermal lensing. For continuous-wave operation,there can be between 0 diopter and at least 15 diopters of positivethermal lensing. An amount of thermal lensing (greater thanapproximately 4 diopters) can be measured by passing a continuous wavelaser probe beam (e.g., from a helium neon laser or laser diode) throughthe gain medium 1-105 (e.g., at an angle) and measuring a relativedisplacement of the probe beam at a distance from the gain mediumbetween “pump-beam-on” and “pump-beam-off” states. A pump-beam-on stateis when the laser diode pump beam is on and exciting the gain medium1-105 for mode-locked lasing of the laser 1-110. Values below 4 diopterscan be difficult to measure accurately, since the relative displacementbecomes small.

Absorption of an optical pump beam in the gain medium 1-105 can causethermal lensing in the gain medium. In embodiments, an amount of thermallensing in the gain medium can be changed by changing an amount of powerin an optical pump beam applied to the gain medium 1-105 (e.g., changingan amount of power from pump module 2-140). Additionally oralternatively, an amount of thermal lensing in the gain medium can bechanged by tuning an optical wavelength of an optical pump beam used toexcite the gain medium 1-105. Tuning of the optical pump beam'swavelength can be performed, for example, by tuning a temperature of alaser diode in the pump module 2-140. Tuning a pump beam's wavelengthcan change an amount of absorption of the optical pump beam in the gainmedium 1-105.

In some implementations, the gain medium 1-105 can comprise neodymiumvanadate (e.g., Nd³⁺:YVO₄), which can provide lasing at 1064 nm. Othersolid state crystals such as, but not limited to, Nd:YAG, Nd:YLF, andCr:Forsterite can be used in other embodiments. In some implementations,a neodymium vanadate gain medium 1-105 can be used to provide lasing at1342 nm alternatively or additionally, provided optical components inthe cavity are designed and coated for lasing at this wavelength. Thegain medium can have a length between 3 mm and 11 mm, in some cases. Insome embodiments, the length of the gain medium can be between 5 mm and9 mm. The neodymium dopant level (atomic %) can be between 0.10% and 1%,in some cases. In some implementations, the dopant level can be between0.10% and 0.50%. In some implementations, the dopant level can bebetween 0.24% and 0.30%. According to some embodiments, the crystallength can be approximately 7 mm and the dopant level can beapproximately 0.27%. The inventors have found that doping levels (atomic%) appreciably higher than 0.3% for lengths of approximately 7 mm candestabilize operation of the laser at higher operating powers (e.g.,induce lasing in higher-order spatial modes, or destabilize or terminatemode locking), which may undesirably require readjusting intracavitycomponents. For example, with 1% doping, mode locking terminated above acertain pump power level, and intracavity optical elements had to bereadjusted to regain mode locking. The transverse dimension ordimensions of the gain medium 1-105 can be any suitable value (e.g.,between 1 mm and 4 mm). The gain medium can be in the form of acylindrical rod, rectangular bar, or any other shape.

End facets of the gain medium 1-105 can be anti-reflection coated forthe lasing wavelength λ₁ (which can be about 1064 nm for neodymiumvanadate) and for the pump wavelength λ_(p) (which can be about 808 nmfor neodymium vanadate), according to some embodiments. In someembodiments, one end facet of the gain medium can be coated with anoutput coupler coating, so that the end facet acts as an end mirror ofthe laser cavity and a separate output coupler 1-111 need not be used.

The gain medium 1-105 can be mounted in a non-adjustable mount (a mountthat provides no fine angular or positional adjustment) in anorientation where end facets of the gain medium have normal vectorsoriented at an angle between about 1 degree and about 3 degrees to theoptical axis 1-125 of the laser cavity. For example, athermally-conductive mount for the gain medium can include a recess inwhich the gain medium 1-105 is placed. The recess can align the gainmedium to the thermally-conductive mount. The thermally-conductive mountcan then register to features on the base chassis 2-105 (e.g., any oneor combination of machined surfaces, pins, screw holes) to align thegain medium at an angle to the optical axis 1-125 of the laser cavity.According to some implementations, the gain medium 1-105 can be cut andoriented in its mount so that it aligns with a favored polarizationintended for lasing. For example, the gain medium 1-105 can be orientedto lase with a linear polarization parallel to the Y axis in FIG. 2-1.

An example of a thermally-conductive mount 3-100 for the gain medium isdepicted in FIG. 3-1A, according to some embodiments. The depicted mount3-100 is designed for a gain medium having a square cross section, butthe mount 3-100 can be designed for other cross-sections such asrectangular, round, oval, or polygonal. According to some embodiments, athermally-conductive mount 3-100 for a gain medium can comprise a firstportion 3-120 and a second portion 3-122 that are configured to bejoined together in a clamping arrangement. For example the first portion3-120 and second portion 3-122 can contain through-holes 3-140 forscrews (not shown) that allow the two portions to be fastened to andplaced in thermal contact with the base chassis 2-105. Screws placed inthe through-holes 3-140 can align the mount 3-100 to the base chassis2-105 and clamp the gain medium 1-105. The first portion 3-120 and thesecond portion 3-122 can be formed from a high-thermal-conductingmaterial such as copper or aluminum, although other materials can beused in other embodiments. The first and second portions can haveinterior faces 3-115 that are arranged to be placed in thermal contactwith the gain medium 1-105. According to some embodiments, there can betrenches or openings 3-130 located at regions of the mount where cornersof the gain medium 1-105 may be located (e.g., when the gain medium1-105 is mounted in the mounting structure 3-100). The trenches oropenings can extend between about 0.5 mm and about 3 mm on either sideof a corner location of the gain medium 1-105. The inventors have foundthat the openings at the corners in the mount 3-100 for the gain medium1-105 can alleviate thermal and mechanical stress that may otherwisecrack the gain medium 1-105 and/or adversely affect the optical modeprofile of the laser.

Another example of a thermally-conductive mount 3-101 for the gainmedium 1-105 is depicted in FIG. 3-1B. The mount 3-101 can include afirst portion 3-121 and a second portion 2-123. The first portion 3-121can include a recess 3-131 machined into the first portion that isslightly oversized compared to the gain medium 1-105 (not shown). Insome implementations, a gain medium (e.g., neodymium-vanadate crystal)can be adhered into the recess 3-131 to interior surfaces 3-116 of therecess with a thermally-conductive adhesive or gel. The oversized recess3-131 can accommodate a thin layer (e.g., less than 400 microns thick)of adhesive or gel that can avoid mechanical stresses from the mount3-101 that would act on the gain medium 1-105. The gain medium can alsobe adhered to a surface of the second portion 3-123 when secured in thelaser cavity 1-110. In some embodiments, the second portion 3-123 can beformed in the base chassis 2-105 (e.g., a platform or other supportingstructure machined into the base chassis 2-105). The first portion 3-121can be connected to the second portion 3-123 with screws, for example.

According to some embodiments, an output coupler 1-111 for a compactmode-locked laser can be a high-quality laser optic having a surfacequality of 10-5 (scratch and dig) and a wavefront error of at most λ/10.One surface of the output coupler 1-111 can be coated with a multi-layerdielectric to provide a reflectivity having a value between about 75%and about 95% for the lasing wavelength λ₁ and allow (with minimalreflectance) transmission of a pump wavelength λ_(p) that is used toexcite the gain medium 1-105. In some embodiments, the lasing wavelengthmay be about 1064 nm and the pump wavelength may be about 808 nm, thoughother wavelengths can be used in other embodiments. In someimplementations, the reflectivity of the output coupler at the lasingwavelength is between 82% and 88%. The inventors have discovered that anoutput coupler within this range of reflectivity provides a desiredamount of output power with stable operation of the laser and providesappropriate amounts of fluence on the saturable absorber mirror 1-119over an operating range of the laser.

A second surface of the output coupler 1-111 (toward the laser output)can be coated with an antireflection coating for both the pumpwavelength and lasing wavelength, and can be oriented at an angle (e.g.,between about 1 degree and about 4 degrees) with respect to thereflective surface of the output coupler. The inventors have found thata small amount of reflection of the lasing wavelength from the output(transmitting) surface of the output coupler 1-111 can appreciably andadversely broaden pulses from the mode-locked laser. According to someembodiments, the coatings on the output coupler are dichroic, so as totransmit with negligible reflection the pump wavelength λ_(p).

According to some embodiments, the output coupler 1-111 can be mountedin a two-axis adjustable mount that provides angular adjustment withrespect to the optical axis 1-125 about two orthogonal axes (e.g., aboutthe Y and X axes in FIG. 2-1). In some embodiments, the output coupler1-111 can be mounted in a non-adjustable mount which can be integratedinto the base chassis 2-105. A non-adjustable mount reduces cost andcomplexity of the compact laser. In yet other embodiments, the outputcoupler 1-111 can be formed as a multilayer optical coating on anend-face of the gain medium 1-105 instead of a separate opticalcomponent comprising a transparent substrate and one or more opticalcoatings.

One example of an integrated non-adjustable mount for an output coupleror other optical component is depicted in FIG. 3-2A and FIG. 3-2B. Theintegrated mount can self-align the optical component to the opticalaxis 1-125 of the laser. An integrated optical mount 3-210 as shown inFIG. 3-2A can comprise an axial trench 3-220 machined or otherwiseformed into the base chassis 2-105 of a mode-locked laser 1-110. Theaxial trench 3-220 can extend in a direction parallel to an optical axisof the mode-locked laser cavity. An integrated optical mount 3-210 canfurther comprise coplanar surfaces 3-230 formed approximately transverseto the axial trench 3-220. The coplanar surfaces can be formed bymachining or milling a short trench in a direction that is approximatelyorthogonal to the axial trench 3-220. In some cases, the coplanarsurfaces 3-230 can be oriented at a small angle, so that backreflections from a mounted optic will be displaced from the optical axisof the laser cavity. At the base of the axial trench 3-220 there can besloped surfaces 3-240 (only one is visible in FIG. 3-2A). The slopedsurfaces 3-240 can be machined, milled, or otherwise formed near thebase of the axial trench and located on opposite sides of the axialtrench 3-220. The sloped surfaces 3-240 can be inclined in a directiontoward the coplanar surfaces 3-230, and provide support for an opticmounted thereon.

An optical component (optic) 3-250 for a mode-locked laser, for example,can be supported by the integrated optical mount 3-210, as depicted inFIG. 3-2B. The optic 3-250 can comprise a cavity mirror, a lens withinthe laser cavity, or the gain medium 1-105, for example. In some cases,the optic 3-250 can be mounted by itself in the integrated optical mount3-210, as depicted in the drawing. In other embodiments, an optic can bemounted within a supporting fixture (e.g., an annular plate, anadjustable mount) that can be placed in the integrated optical mount3-210.

According to some embodiments, an optical component 3-250, or supportingfixture, can include a flat surface that registers to and rests againstthe coplanar surfaces 3-230 of the integrated optical mount 3-210. Theoptic or fixture can be retained in the integrated mount by a compliantretaining device (e.g., an O-ring mounted on a bar that can be fastenedto the base chassis, a flexible plastic bar or arm, etc.). The compliantretaining device can contact a top edge of the optic 3-250 or supportingfixture, and can exert forces on the optic or fixture in directionstowards inclined surfaces 3-240 and the coplanar surfaces 3-230. A loweredge of the optic 3-250 or supporting fixture can contact points on theinclined surfaces 3-240. The inclined surfaces 3-240 can also provide aforce against the optic or fixture having a component that is directedin part toward the coplanar surfaces 3-230. The contact points at theinclined surfaces 3-240 and forces directed toward the coplanar surfaces3-230 can self-align the optic or fixture to a desired orientation andlocation within the laser cavity. In some implementations, an optic orsupporting fixture can be bonded in the integrated optical mount (e.g.,with an adhesive) in an aligned orientation.

One or more integrated optical mounts 3-210 can be formed in a basechassis of a mode-locked laser 1-110, according to some embodiments. Insome cases, an axial trench 3-220 can extend through several integratedoptical mounts, as depicted in FIG. 3-2A. Among the advantageousfeatures of an integrated optical mount are a lowering of themode-locked laser's optical axis. This can reduce effects of mechanicalvibrations that might otherwise couple into and be amplified by opticalmounts extending from a surface of the base chassis, and can reduceeffects of thermal expansion (e.g., slight warping of the base chassis2-105) that might otherwise be amplified by motion of optical mountsextending from a surface of the base chassis.

Referring again to FIG. 2-1, the inventors have discovered that changinga distance between the output coupler 1-111 and the gain medium 1-105can change the FWHM value of the mode-locked pulse temporal profile(also referred to as pulse duration). Mode-locking of the laser can beachieved with the distance between the output coupler 1-111 and the gainmedium 1-105 varied between 0 mm and 10 mm, and the pulse duration canbe varied between approximately 9 picoseconds and approximately 38picoseconds over this range of distances by selecting differentdistances to obtain different pulse durations. According to someembodiments, the distance between the output coupler 1-111 and the gainmedium 1-105 is set between 4 mm and 8 mm.

The inventors have also discovered that stable and efficient operationover a range of average lasing powers is achieved when the intracavitybeam waist of the laser at the output coupler 1-111 is between 100microns and 180 microns. The value of the beam waist at the outputcoupler 1-111 is determined in part by intracavity optics, such ascurved mirror 2-117, by distance of the output coupler to the curvedmirror, and by the pump beam waist in the gain medium 1-105. Accordingto some embodiments, the beam waist of the lasing wavelength in the gainmedium can be significantly smaller that the pump beam waist in the gainmedium 1-105. For example, the beam waist for the lasing wavelength inthe gain medium can be between 100 microns and 150 microns in the gainmedium, and a smallest waist for the pump beam can be between 180microns and 250 microns, wherein the pump beam may not be fullysymmetric about its optical axis. The value of the beam waist at theoutput coupler 1-111 and in the gain medium 1-105 may also be affectedby the focal length of the second curved mirror 2-127 and its distanceto the saturable absorber mirror 1-119. Having a smaller beam waist forthe lasing beam of the mode-locked laser 1-110 than the laser diode pumpbeam can improve stability of the mode-locked laser operation (e.g.,make the laser less susceptible to power and mode-locking fluctuationsdue to relative motion of the laser beam and laser diode pump beam inthe gain medium 1-105. The term “beam waist” is used to refer to thespatial extent at which the laser beam intensity falls from a peak valueto a 1/e² value on opposite sides of the beam. A round beam may becharacterized by a single beam waist. An elliptical beam may becharacterized by two beam waists: one for the beam's minor axis and onefor the beam's major axis.

At an opposite end of the laser cavity, a saturable absorber mirror(SAM) 1-119 be mounted. Referring to FIG. 3-3, the SAM can comprise amultilayer semiconductor structure 3-312 that exhibits nonlinear opticalabsorption (e.g., a multiple quantum well) and a high reflector 3-330formed on a substrate 3-305. The nonlinear optical absorption can inducepassive mode locking in the laser. For example, the SAM can exhibithigher absorption and loss at low optical intensities, and can bleach orexhibit little absorption and less loss at high optical intensities. Thesemiconductor structure 3-312 can be spaced from the high reflector3-330 in the SAM so that the semiconductor structure is located atapproximately a peak intensity of an optical standing wave created bythe optical field incident on and reflected from the high reflector3-330. An example of a SAM is part number SAM-1064-5-10ps-x availablefrom BATOP Optoelectronics GmbH of Jena, Germany. Because of the SAM'snonlinear optical absorption, the laser preferentially operates in apulsed mode of operation (passively mode locked) since the highintensities of the optical pulses experience less loss in the cavitythan lower intensity, continuous-wave operation of the laser.

In some implementations, a SAM 1-119 can be mounted on a rotating and/ortransverse-positioning mount, so that the SAM's surface can be moved ina direction transverse to the optical axis 1-125 (the Z axis in thedrawing). Should the SAM become damaged, the SAM can be moved and/orrotated so that the intracavity beam is focused onto an undamaged regionof the SAM. In some cases, the SAM 1-119 can be mounted on a mount thatprovides angular adjustment, e.g., to aid in alignment of the lasercavity.

In other embodiments, the SAM can be mounted on a non-adjustable mount2-119. The non-adjustable mount can include a thermal conductor such asaluminum or copper that dissipates heat from the SAM to the base chassis2-105 (not shown in the drawing). In some embodiments, the SAM mount2-119 can comprise a plate of aluminum or copper or any suitablethermally-conductive material to which the SAM is adhered with athermally-conductive adhesive. In some implementations, the SAM can beadhered to a copper foil on a piece of a printed circuit board, which isused as the SAM mount 2-119. The SAM mount can be attached to a machinedsurface in the base chassis or a surface of a fixture attached to thebase chassis with one or more screws that allow the same to be roughlyaligned to the optical axis 1-125 of the laser. For example, the SAMmount can be crudely positioned by hand in X and Y directions whensecured to the base chassis, but otherwise not provide for fine angularadjustment (e.g., in two degrees of freedom) of the SAM's surface withrespect to an optical axis of an intracavity beam of the mode-lockedlaser that is incident on the SAM. Other optical components in the lasercavity can be used to adjust the incident angle and position of theoptical axis on the SAM. By mounting the SAM 1-119 on a fixed mount,cost and complexity associated with a multi-axis/multi-angle adjustmentmount can be eliminated.

According to some embodiments, the SAM can be formed from agallium-arsenide semiconductor composition. The SAM can be cut from alarger substrate or wafer, and can be square in shape with a maximumdimension across the face of the SAM between 1 mm and 3 mm. A relaxationtime of the SAM's absorption can be between 10 ps and 30 ps. Anon-saturated absorption of the SAM can be between 2% and 6%. Themodulation depth of the SAM can be between 60% and 74% of the SAM'snon-saturated absorption. In some implementations, the relaxation timeis approximately 25 ps and the non-saturated absorption is approximately4%. Such a SAM 1-119 can support mode-locked lasing with pulse durationsbetween 12 ps and 20 ps. A saturation fluence of the SAM can be about 70microJoules/cm² (μJ/cm²), in some embodiments.

The inventors have recognized and appreciated that the optical fluenceon the SAM from the intracavity laser beam should be kept below 2.5milliJoules/cm² (mJ/cm²) for prolonged operation of a gallium-arsenideSAM. At values equal to 5 mJ/cm² or higher, the SAM may damage. In someimplementations, the fluence on the SAM can be kept below about 10 timesthe saturation fluence of the SAM. The fluence on the SAM can becontrolled by controlling the beam waist at the SAM (e.g., with a curvedmirror 2-127 located in the laser cavity) and by controlling theintracavity power with the choice of reflectivity of the output coupler1-111. According to some embodiments, a beam waist at the SAM is between80 microns and 120 microns when the output coupler reflectivity isbetween 82% and 88%.

Between the output coupler 1-111 and the SAM 1-119, there can be aplurality of mirrors that fold the optical axis of the laser cavitymultiple times. Some of these mirrors (e.g., mirrors 2-115, 2-120,2-121, 2-122, 2-123, 2-124, 2-125 referring to FIG. 2-1) can have flatsurfaces and be mounted in non-adjustable mounts. According to someembodiments, two of the mirrors 2-117, 2-127 can have curved surfacesand comprise a focusing reflector; In some cases, another type offocusing optic (e.g., a lens or compound lens) can be used instead offocusing reflectors for mirrors 2-117, 2-127 (e.g., if the intracavitybeam is not folded at the location of the mirrors 2-117 or mirror2-127). For flat and curved mirrors that are used to fold the opticalaxis of the laser, the reflectivity of the mirror can be very high forthe lasing wavelength at the angle of incidence for which the mirrorwill be used. For example, the reflectivity for such a mirror can begreater than 99% in some cases, and yet greater than 99.5% in somecases. The surface quality of one or more of the folding mirrors can beat least 10-5 (scratch and dig) and a wavefront error can be at mostλ/10. In some cases, the surface quality of one or more of the foldingmirrors can be at least 40-20 (scratch and dig) and a wavefront errorcan be at most λ/10. A higher value for scratch-dig surface quality cansignificantly reduce the cost of the folding mirrors.

In some implementations, at least one of the mirrors (e.g., mirror2-124) can fold the intracavity beam multiple times for a single transitfrom the gain medium 1-105 to the SAM 1-119. For the exampleconfiguration shown in FIG. 2-1, a bounce sequence for an optical pulse1-120 travelling from the gain medium 1-105 to the SAM 1-119 is asequence of reflections from mirrors 2-115, 2-117, 2-120, 2-121, 2-122,2-123, 2-124, 2-125, 2-124, 2-127, 2-124, and then to the SAM 1-119. Inthis sequence, one of the intracavity mirrors 2-124 is used for multiplereflections and the angle of incidence is reversed in sign on thismirror for at least two reflections as the beam travels from one end ofthe laser cavity to the other end. For example and referring to FIG.2-1, the first angle of incidence is in the +Z direction and the secondangle of incidence on mirror 2-124 is in the −Z direction as the beamtravels from the output coupler 1-111 to the SAM 1-119. After reflectingfrom the SAM 1-119, the pulse will then return in the reverse bouncesequence to the gain medium. By having multiple folds of the opticalaxis within the compact laser module, the cavity length can be extendedto obtain a pulse repetition rate below 200 MHz and as low as 50 MHz.The pulse repetition rate will depend upon the length of the lasercavity, which is determined in part by the number of bounces betweenmirrors in the cavity and the distances between the mirrors. Accordingto some embodiments, the pulse repetition rate can be changed byrelocating mirrors and adding or removing mirrors within the cavitybetween the first curved mirror 2-117 and the second curved mirror 2-127to increase or decrease the optical path length between the outputcoupler 1-110 and saturable absorber mirror 1-119. Because theintracavity beam is approximately collimated between the first curvedmirror 2-117 and the second curved mirror 2-127, changes to pulserepetition rate can be made more easily than if the beam were notcollimated in this region. In some implementations, extra integratedoptical mounts can be formed in the base chassis for relocating mirrorsto obtain different pulse repetition rates.

As noted above, the inventors have recognized and appreciated that pulserepetition rates below 200 MHz and as low as 50 MHz are desirable formassively-parallel analysis of samples on a bio-optoelectronic chip.However, using multiple mirrors, with some mirrors used multiple times,requires a very high degree of stability of the mirrors with respect toeach other to maintain stable mode-locked lasing over periods of hours.Integrated mounting of the mirrors against supporting surfaces in a basechassis 2-105 that includes strengthening ribs can achieve the requisitestability of the mirrors and stable mode-locking operation.

An example of a non-adjustable mount for a folding mirror is shown inFIG. 3-4. According to some embodiments, the mount can be machined orotherwise formed into the base chassis 2-105. The mount can comprise asupporting and aligning wall 3-410 located adjacent to two slopedsurfaces 3-424 that are spaced apart. The sloped surfaces can be formedon two protrusions 3-420, according to some embodiments. In someimplementations, there can be a single sloped surface. The slopedsurface or surfaces can be inclined toward the aligning wall 3-410, asillustrated in the drawing. There can be one or more threaded holes3-430 adjacent to the wall. An optical component (e.g., a flat mirror orcurved mirror) can be placed on the sloped surface or surfaces 3-424with a back side resting against the aligning wall 3-410. A clampingcomponent (not shown) having a pliable or flexible component can besecured via the threaded hole 3-430 or holes to retain the opticalcomponent against the aligning wall.

The aligning wall 3-410 can be machined in the base chassis 2-105 with aselected orientation, so that an optical component held against thealigning wall 3-410 will be approximately aligned at desired angles withrespect to a planned optical axis of the laser beam through the lasercavity. The inventors have recognized and appreciated that aligningwalls 3-410 can be formed to a high degree of angular accuracy bymachining for angles lying within a plane parallel to the base chassis(e.g., for angles that define incident and reflection angles of thelaser beam in an XZ plane in FIG. 2-1). However, the machining accuracyof forming the aligning walls 3-410 is appreciably less for angles thatwould deflect the laser beam out of a plane parallel to the basechassis. Accordingly, one of the mirror mounts between the gain medium1-105 and SAM 1-119 can include an angular adjustment (one degree offreedom) to accommodate for manufacturing errors that would causedeflection of the laser beam out of a plane parallel to the basechassis. According to some embodiments, the mirror mount having a singledegree of freedom is located between one-quarter and three-quarter ofthe distance between the gain medium and SAM.

In some implementations, one folding mirror 2-115 can be configured tocontrol polarization of radiation within the cavity and allow monitoringof pump-beam radiation (indicated as the heavy dashed line in FIG. 2-1).For example, the folding mirror 2-115 can be coated to reflect spolarization (polarization that is out of the plane of the base chassis,in the Y direction) with a high reflectivity greater than 99%, or evengreater than 99.5% in some cases, and to have a lower reflectivity forthe orthogonal p polarization, so that lasing in the p polarization isprevented. In some cases, the folding mirror 2-115 can be a polarizingbeam splitter that transmits more than 20% of the p polarization andreflects the s polarization with high reflectivity. The folding mirror2-115 can additionally transmit most or nearly all of the pump-beamradiation to a photodetector 2-116 located behind the mirror. Thefolding mirror can include a dichroic coating to allow transmission ofthe pump-beam radiation, in some embodiments. In other embodiments, adichroic coating may not be used, and the coating for the lasingwavelength may allow adequate transmission of the pump-beam radiationthrough the folding mirror 2-115 for detection. An output from thephotodetector 2-116 can be provided to the PCB 2-190 for signalprocessing and/or transmission to an external signal processor.

In some embodiments, two curved mirrors 2-117, 2-127 can be designed andlocated within the laser cavity to obtain desired beam waist sizeswithin the gain medium 1-105 and the SAM 1-119. A first curved mirror2-117 can be located in a first portion of the laser cavity near thegain medium 1-105. A second curved mirror 2-127 can be located in asecond portion of the laser cavity near the SAM 1-119. At least betweenthe curved mirrors, there can be a plurality of folding mirrors thatfold the optical axis of the laser and extend the laser cavity length ina cavity length extending region. There can additionally be a mirror2-124 between curved mirror 2-127 and the SAM 1-119 that folds theintracavity laser beam multiple times to extend the length of the cavityin the cavity length extending region. For example, curved mirror 2-127and mirror 2-124 can fold the intracavity beam three times onimmediately successive bounces from these two reflectors, as indicatedin FIG. 2-1.

According to some embodiments, the first curved mirror 2-117 can be aspherical reflector and have a focal length f_(l) between 240 mm and 260mm. A tolerance on the focal length for this reflector can be ±1% of thefocal length. The inventors have found that the first curved mirror2-117, with a focal length of approximately 250 mm, can be placedbetween 230 mm and 310 mm from the output coupler 1-111 and stablemode-locked operation having different characteristics can be obtained.According to some embodiments, the first curved mirror can be locatedbetween 280 mm and 300 mm from the output coupler to obtain stablemode-locked operation over a large range of operating powers of thecompact laser module. In this configuration, the gain medium 1-105 canbe located between 4 mm and 8 mm from the output coupler. The focallength of the first curved mirror 2-117 and its location with respect tothe gain medium 1-105 and output coupler 1-111, and the focal length ofthe second curved mirror 2-127 and its location with respect to the SAM1-119 can determine the beam waist of the intracavity beam in the gainmedium.

A focal length of the first curved mirror 2-117 may have other values inother embodiments. For example, a shorter focal length f_(l)<230 mm canbe used for a more compact mode-locked laser that operates at lowerpowers. In embodiments, the output coupler 1-111 can be placed adistance d₁ from the first curved mirror 2-117 that is in a range ofvalues within 30% of the focal length f_(l) (e.g.,0.7f_(l)<d_(l)<1.3f_(l)). In some cases, 0.9f_(l)<d_(l)<1.3f_(l).

In some implementations, the first curved mirror 2-117 can be mounted inan adjustable mount that provides only two degrees of freedom foradjusting orientation angles (in-plane, and out-of-plane angles) of themirror with respect to the optical axis of the laser. An adjustablemount can allow an operator to finely adjust the position (one or moreof X, Y, Z) and/or orientation (pitch and/or yaw with respect to theoptical axis of the incident intracavity beam) of the optical componentwhile the laser is lasing, so that operation of the laser can be tunedfor stability, beam quality, output power, and/or pulse characteristics.Fine tuning can be achieved by micrometers and/or finely-threaded screwadjustments on mirror mounts, for example.

Providing only two degrees of freedom for the first curved mirror 2-117and only one degree of freedom for a folding mirror (e.g., mirror 2-123)as the only adjustments for aligning the laser cavity in real time whilethe laser is lasing can reduce cost and complexity of the compactmode-locked laser module. In other cases, the mirror mount for the firstcurved mirror 2-117 can include additional degrees of freedom foradjusting the position of the mirror, for example. According to someembodiments, adjustments can be made to the pump module 2-140 afteradjusting curved mirror 2-117 to align or re-align the pump beam andincrease output power from the mode-locked laser.

A second curved mirror 2-127 can be a spherical reflector and have afocal length f₂ between 240 mm and 260 mm. A tolerance on the focallength for this reflector can be ±1% of the focal length. The inventorshave found that the second curved mirror 2-127, with a focal length ofapproximately 250 mm, can be placed between 260 mm and 290 mm from theSAM 1-119 and stable mode-locked operation having differentcharacteristics can be obtained. According to some embodiments, thesecond curved mirror can be located between 270 mm and 285 mm from theSAM 1-119 to obtain stable mode-locked operation over a large range ofoperating powers of the compact laser module. The focal length of thesecond curved mirror 2-127 and its location with respect to the SAM1-119 can determine the beam waist of the intracavity beam at the SAM1-119 and also affect the beam-waist at the gain crystal.

A focal length of the second curved mirror 2-127 may have other valuesin other embodiments. For example, a shorter focal length f₂<240 mm canbe used for a more compact mode-locked laser that operates at lowerpowers. In embodiments, the SAM 1-119 can be placed a distance d₂ fromthe second curved mirror 2-127 that is in a range of values within 20%of the focal length f₂ (e.g., 0.8f₂<d₂<1.2f₂). In some cases,f₂<d₂<1.2f₂.

The second curved mirror 2-127 can be mounted in a non-adjustable mount,as described above in connection with FIG. 3-4, for example, to reducecost and complexity of the laser module. As described above, all of thereflective components in the laser cavity (except the first curvedmirror 2-117 and the folding mirror 2-123) can be mounted inself-aligning, non-adjustable mounts. Further, the first curved mirror2-117 can have only two degrees of freedom for angular adjustments andthe folding mirror 2-123 can have only one degree of freedom for angularadjustment. The inventors have discovered that the mode-locked lasercavity can be aligned for stable operation for long periods of timeusing only these three adjustments, according to some embodiments. Forexample, the first curved mirror 2-117 can be used to steer a beam fromthe gain medium 1-105 to the SAM 1-119, which is mounted in a fixedlocation to receive the beam. Any out-of-plane deviations (in the ±Ydirections in FIG. 2-1) can be accommodated by adjusting the singledegree of angular adjustment on folding mirror 2-123. If the SAM 1-119does not receive the intracavity beam at normal incidence so as toreflect the beam back along the same path, the angle of incidence on theSAM can be adjusted by translating the intracavity beam on the secondcurved mirror 2-127. Since the SAM 1-119 is nearly at the focus of thesecond curved mirror, a translation of the beam on this mirror altersthe incidence angle at the SAM. The intracavity beam can be translatedacross the surface of the second curved mirror by making angularadjustments to the first curved mirror 2-117. Adjustments can be made tothe first curved mirror until the intracavity beam is reflected back onitself from the SAM 1-119.

The inventors have discovered that the spot size of the intracavitylaser beam on the SAM can be more sensitive to changes in distancebetween the first curved mirror 2-117 and the laser's output coupler1-111 than to changes in distance between the second curved mirror 2-127and SAM 1-119. This result relates to the extended cavity length betweenthe first curved mirror 2-117 and the second curved mirror 2-127. Thisextended cavity length can be more than half the length of the lasercavity, throughout which the intracavity laser beam can be approximatelycollimated. Changes in the distance between the curved mirror 2-117 andoutput coupler 1-111 can affect collimation in the extended cavity,which can amplify changes in beam size at the second curved mirror2-127. The amplification in turn affects the spot size in the SAM 1-119more strongly than changes in distance between the second curved mirror2-127 and SAM 1-119. Accordingly, the position of the first curvedmirror 2-117 can be used to adjust the fluence on the SAM 1-119. In someembodiments, the amplification effect can be reduced by increasing thefocal length of the second curved mirror 2-127.

When the laser cavity is aligned and configured as described above, suchthat a beam waist in the gain medium 1-105 is between 100 microns and150 microns, and the beam waist at the SAM 1-119 is between 80 micronsand 120 microns, the inventors have discovered that the laser cavitysatisfies a “stability criterion” for optical resonators (a conditionknown to those skilled in the art of lasers) that spans a change from 0diopter to 15 diopters of thermal lensing effects in the gain medium1-105 and for focal length errors of the two curved mirrors 2-117, 2-127of ±1%. At high optical powers, the gain medium 1-105 can acquireappreciable heat from the pump radiation, and the heated gain medium cancreate an optical lens (also referred to as thermal lensing) that has afocusing power (diopter) that is dependent upon the temperature of themedium. For optically-pumped, high-power lasers, the changes due to thisthermal lensing can destabilize the laser and extinguish lasing forincreases in pump power by 50% from an initial stable operating point.The inventors have observed that the compact mode-locked laser module1-108 maintains stable mode-locking operation for variations in pumppower from 2 Watts to 8 Watts, an increase of 300% in pump power from aninitial stable operating point. The range of stability for the lasercavity is surprisingly large, and allows the compact mode-locked laserto be operated over a large range of intracavity and output powers. Forexample, the average output power from the laser can vary between 350milliwatts and 3.5 Watts over this range of pump power, while the FWHMpulse duration remains between 12 picoseconds and 18 picoseconds. Thisoutput can be frequency doubled to produce pulses of a same duration ata wavelength of 532 nm, for example, with average power levels between100 milliwatts and 1.5 Watts.

According to some embodiments, there can be optical components mountedwithin the laser cavity to help stabilize operation of the mode-lockedlaser and/or improve beam quality of the mode-locked laser. For example,a spatial mode filter 2-118 can be located in the laser cavity andconfigured to prevent lasing in higher-order spatial modes. The modefilter 2-118 can comprise an aperture of any suitable shape (e.g.,round, oval, crescent shaped, square, rectangular, polygonal, etc.). Theaperture can be mounted in a non-adjustable mount, or can be mountedsuch that it can be moved in directions transverse to the intracavitybeam's optical axis. The size of the aperture can be adjustable in somecases (e.g., an iris). In various embodiments, the aperture constrainslasing operation to the lowest-order transverse spatial mode of thelaser cavity, which can improve stability of mode locking.

Beam steering components can be included in the laser module 1-108 insome embodiments for dynamic stabilization and alignment. For example,one or more anti-reflection coated laser windows or optical flats 2-128that can be rotated at an angle with respect to the intracavity beam canbe operated automatically by an actuator 2-162 to translate and/orchange an incident angle of the intracavity beam on the SAM 1-119. Therecan be mechanical linkage 2-164 between an actuator and laser window anda pitch or yaw mount for the laser window that enable automated pitch oryaw adjustments to the laser window 2-128. The actuator 2-162 cancomprise a stepper motor, piezoelectric transducer, capacitivetransducer, or any other suitable actuator.

Rotation of an intracavity laser window will shift laterally theoutgoing beam from the laser window in the direction of rotation. Theamount of lateral shift can be determined by applying Snell's law to thetwo interfaces of the laser window. If the laser window is locatedbetween the second curved mirror 2-127 and the SAM 1-119, then rotationof the laser window will mainly translate the intracavity beam on theSAM. Rotation of such laser window can be used to extend the lifetime ofthe SAM by moving the intracavity beam across the SAM. A scanning motionmy reduce fatigue of the SAM, or if the SAM has been damaged the beamcan be moved away from the damaged spot. If the laser window 2-128 islocated before the second curved mirror 2-127 as depicted in FIG. 2-1,then rotation of the laser window will mainly change the incident angleof the intracavity beam on the SAM. Rotation of such laser window can beused to dynamically align or realign the laser cavity to obtain and/ormaintain stable mode-locked operation.

Signals that indicate laser performance and that can be used forautomatically adjusting intracavity beam-steering components can includeany one or combination of pump power (detected with photodetector 2-116or a pump photodetector (not shown) that is mounted in the pump module),laser power and/or pulse characteristics (detected with a laser outputphotodetector 2-154, which can be sensitive to the lasing wavelength),and second-harmonic power (detected with a doubled-output photodetector2-182). The signal or signals can be provided to circuitry on PCB 2-190for processing and generation of feedback control signals to operate oneor more actuators 2-162. In some embodiments, one or both of the laseroutput photodetector 2-154 and doubled-output photodetector 2-182 can bemounted on the PCB 2-190 and received radiation through a hole and/orwindow (not shown) located in a side of the mode-locked laser module1-108. In some implementations, rotation of an intracavity beam-steeringcomponent can be automated to fine tune cavity alignment and/or change aposition of the intracavity beam on the SAM 1-119 based on one or morefeedback signals.

According to some embodiments, cavity alignment can be obtainedadditionally or alternatively by inducing asymmetric thermal gradientsin the gain medium 1-105. Asymmetric thermal gradients can affectthermal lensing and alter the refractive index within the gain medium1-105 in such a way to cause small angular deflections in theintracavity laser beam as it passes through the gain medium 1-105. Insome implementations, one or more temperature-controlling devices (e.g.,resistive heating elements, TEC coolers, or a combination thereof) canbe coupled to one or more sides of the gain medium. According to someembodiments, the gain medium 1-105 can have two to fourindependently-operable, temperature-controlling devices (not shown inFIG. 2-1) thermally coupled to two to four faces (four longitudinaledges) of the gain medium. Thermal coupling can comprise thermal epoxyor indium foil located between a temperature-controlling device and faceof the gain medium 1-105. A temperature-controlling device can alsoinclude thermal coupling to a heat sink (such as the laser block) on anopposite side of the temperature-controlling device. In some cases,operation of one or more of the temperature-controlling devices canprovide beam deflection transverse to the optical axis 2-111. Byselectively altering temperatures at the temperature-controllingdevices, the intracavity laser beam can be steered and re-aligned. Insome cases, one or more intracavity laser windows 2-128 can be adjustedin tandem with thermal beam steering in the gain medium to repositionthe intracavity beam on the SAM, for example, and/or maintain stablemode-locked operation of the laser.

The inventors have recognized and appreciated that average power and/orspectral characteristics of the mode-locked laser can be determinativeof stable, mode-locked operation. For example, if the laser's averagepower during mode-locked operation falls below a certain value, theremay not be enough nonlinear optical absorption in the SAM 1-119 tosupport mode locking. The laser may then Q-switch and damage the SAM1-119. In some cases, rapid fluctuations of the laser's average outputpower may indicate that the laser is Q-switching in addition to modelocking, which can damage the SAM 1-119. In some embodiments, at leastone sensor 2-154 (e.g., a photodiode) can be included and arranged tosense optical power produced by the laser 1-110 and/or output pulse ormode-locking characteristics of the laser. For example, a signal from afirst sensor 2-154 can be spectrally analyzed to detect sidebands nearthe mode-locking frequency, which can indicate the onset of Q-switchingand/or instabilities in the mode-locked pulse train of the laser 1-110.A second sensor (not shown) can detect average optical power produced bythe laser 1-110. If the sensed average laser power drifts below a presetlevel and/or if sidebands or power fluctuations are detected by thefirst sensor 2-154, an automated cavity alignment routine can beexecuted to recover power and/or the laser can be shut off forservicing. In some cases, sidebands that indicate instabilities in themode-locked pulse train are due to lasing of higher-order spatial cavitymodes. Such instabilities can be corrected by adjusting an intracavityspatial mode filter 2-118 automatically or manually, for example.According to some embodiments, one or more sensors 2-154 that aresensitive to the lasing wavelength can be mounted on PCB 2-190.

In some cases, additional signals can be processed to analyze laserbehavior. For example, the pump power can be evaluated with a pump powersensor 2-116 (which can be a photodiode or other suitable photodetector)in conjunction with the average power level from the laser. In someembodiments, the amount of frequency-doubled power can be monitored withsensor 2-182 (which can be a photodiode or other suitable photodetector)additionally or alternatively. For example, a reduction in averagefrequency-doubled power while the average laser power remains nearlyconstant could indicate changes in mode-locked pulse length, or aproblem with the frequency-doubling optical components.

In operation, a mode-locked laser 1-110 that employs Nd³⁺:YVO₄ as thegain medium and arranged as described above can produce pulses at 1064nm having a FWHM value of approximately 15 ps. The pulse extinguishes byapproximately 80 dB within 100 ps from the peak of the pulse. The pulserepetition rate is approximately 67 MHz, and the average power of themode-locked laser at the fundamental wavelength can be varied from 350mW to 3.5 W. The conversion efficiency to a frequency-doubled wavelength(described further below) can be as high as 30% in some cases, so thatpulses at 532 nm can be produced with average output powers between 100mW and 1.5 W. In some cases, the conversion efficiency can be as high as35%. The AC power required to operate the laser is less than about 20Watts. The laser is compact, occupies a volume of less than 0.1 ft³,weighs slightly less than 2 kilograms, and can be readily incorporatedas a module into a portable analytic instrument, such as a table-topinstrument for sequencing DNA.

Alternative Configurations for the Laser Cavity

Although the compact mode-locked laser module 1-108 described above usesmultiple mirrors that extend the cavity length and reduce the pulserepetition rate, other embodiments can use other optical componentsadditionally, or alternatively, to extend the cavity length. Someexamples of optical delay elements are depicted in FIG. 3-5A throughFIG. 3-5D. According to one embodiment, an optical delay element 3-510can comprise an argyle block, as depicted in the plan view of FIG. 3-5A.The argyle block can comprise a first right-angle prism 3-520 and asecond right-angle prism 3-522. According to some embodiments, theperpendicular side faces of the prisms can be uncoated, though in otherembodiments the perpendicular faces can include high-reflectivecoatings. In some implementations, a length of a perpendicular face onone of the prisms can measure between about 20 mm and about 60 mm. Eachprism can be formed of any suitable optical quality glass, for exampleBK-7 or fused silica. For high thermal stability, the delay element canbe formed from an ultra-low expansion glass such as ULE, available fromCorning. The side faces of the prisms can be polished to be of highoptical quality, for example, having a wavefront error of λ/10 or betterand a surface quality of 10-5, for example.

The first prism 3-520 and second prism 3-522 can be offset and adheredtogether, as depicted in the drawing. The prisms can be adhered viaoptical bonding or using an optical adhesive. In some implementations,the optical delay element 3-510 can be formed from a single piece ofglass by cutting and polishing. An intracavity laser beam 3-101 canenter through a first port of the delay element and be reflectedinternally along a circuitous optical path, depicted as the dotted line,before exiting a second port of the argyle block.

According to some implementations, a delay element can be double-passedto double the optical path length provided by the delay element. Forexample, an output beam from a single-pass output port of the delayelement can be retroflected with a spatial offset back through the delayelement, so that the return beam exits the input port but is displacedfrom the input beam 3-101, which can be received from a first portion ofthe laser cavity. The displaced output beam can be directed to a secondportion of the laser cavity.

Another embodiment of an optical delay element 3-512 is depicted in FIG.3-5B. According to some embodiments, the optical delay element cancomprise a single optical block that is formed in a rectangular shape.The delay element 3-512 can comprise perpendicular edge faces 3-530 thatreflect an intracavity beam back-and-forth within the delay element, asdepicted in the drawing by the dotted line. The delay element canfurther include two polished faces that provide an entry port 3-532 andexit port 3-534 for the delay element. The perpendicular side faces canbe uncoated in some embodiments, or coated with a high-reflectivecoatings (e.g., multilayer coatings) in other embodiments. In someimplementations, a maximum length of an edge of the delay element canmeasure between about 20 mm and about 60 mm. The thickness of the block,measured in a direction into the page, can be between about 5 mm andabout 20 mm. The delay element 3-512 can be formed of any suitableoptical quality glass, as described above. The reflective edge faces canbe polished to be of high optical quality, for example, having awavefront error of λ/10 or better and a surface quality of 10-5, forexample. The delay element 3-512 can be doubled-passed to increase theoptical path length within the laser cavity.

FIG. 3-5C depicts yet another embodiment of an optical delay element3-514. According to some embodiments, the delay element can comprise apair of planar mirrors M₁, M₂ that are spaced a distance D apart attheir centers and inclined at a slight angle α with respect to eachother. Each mirror M₁, M₂ can have a length L. The spacing D between themirrors M₁, M₂ can be between about 10 mm and about 50 mm, according tosome embodiments. The length L of the mirrors M₁, M₂ can be betweenabout 20 mm and about 60 mm, according to some embodiments. The angle αcan be between about 0° and about 10°, according to some embodiments.The height of the mirrors M₁, M₂, measured along a direction into thepage, can be between about 5 mm and about 20 mm. The mirrors M₁, M₂ canbe formed of any suitable optical quality glass, as described above. Thereflective surfaces of the mirrors M₁, M₂ can be polished to be of highoptical quality, for example, having a flatness of λ/10 or better and asurface quality of 10-5, for example. The reflective surfaces can becoated with high-quality, high-reflective, multilayer coatings and havea reflectivity greater than about 99.5% in some implementations. In someembodiments, the reflectivities can be greater than about 99.9%. Anintracavity beam 3-101 entering the mirror pair in a first directionwill undergo multiple reflections, dependent upon the incident angle andangle α between the mirrors M₁, M₂.

Another embodiment of an optical delay element 3-516 is depicted in FIG.3-5D. This embodiment may comprise a solid block analog to theembodiment depicted in FIG. 3-5C. According to some implementations, anoptical delay element 3-516 can comprise a solid block of opticalmaterial having five surfaces as depicted in the drawing. Two surfaces3-534 can be inclined at a slight angle α with respect to each other.These surfaces can include high reflective coatings to reflect anintracavity beam 3-101 back-and-forth between the surfaces along adotted path as indicated in the drawing. The delay element 3-516 canfurther include two uncoated or anti-reflection coated surfaces 3-532that provide an entry port and exit port to and from the delay element.According to some embodiments, the delay element can be arranged so thatthe intra-cavity laser beam 3-101 enters and exits the delay element atBrewster's angle. The delay element 3-516 can be formed of any suitableoptical quality glass, as described above. The reflective surfaces 3-534can be polished to be of high optical quality, for example, having aflatness of λ/10 or better and a surface quality of 10-5. The reflectivesurfaces can be coated with high-quality, high-reflective, multilayercoatings and have a reflectivity greater than about 99.5% in someimplementations. In some embodiments, the reflectivities can be greaterthan about 99.9%.

An advantage of solid-block delay elements 3-510, 3-512, 3-516respectively depicted in FIG. 3-5A, FIG. 3-5B and FIG. 3-5D is thatthese elements do not require as careful alignment when inserted intothe laser cavity as would be required for multi-component delay elementssuch as the two mirrors of FIG. 3-5C or the multiple flat mirrors shownin FIG. 2-1. However, solid block components can require a larger numberof reflections from mirror surfaces for pulse repetition rates below 200MHz, and will require more precision during a manufacture. As a result,the cost of the solid-block delay elements can be high. By usingintegrated non-adjustable mounts in a single-piece base chassis 2-105and using one or two adjustable mounts to accommodate for machiningerrors in the non-adjustable mounts, as described above, lower costmirrors can be used to provide a desired optical delay. An advantage ofthe multi-mirror delay element is that the cavity length can be changedmore readily and flexibly by changing the position of one or morecavity-folding mirrors to redefine the laser cavity.

Although thermal effects within the gain medium 1-105 can be used tosteer and align the intracavity beam, as described above, the inventorshave recognized and appreciated that thermal heating effects and/ormechanical stresses on optical elements within the laser cavity can be asignificant factor that can undesirably influence the performance of acompact, mode-locked laser. Thermal heating can arise at the pump module2-140 and the gain medium 1-105 when mode-locked laser 1-110 is operatedat average power levels for the fundamental lasing wavelength over 250mW, for example. In regard to the gain medium 1-105, the inventors haverecognized and appreciated that additional care must be taken whenmounting a gain crystal such as neodymium vanadate. A mount should allowfor heat dissipation, and yet avoid mechanically stressing the crystal.The mount with relief cuts at vertices, shown in FIG. 3-1A, can allowfor heat dissipation and avoid undesirable stresses on the crystal.Additionally or alternatively, the use of a thermally-conductiveadhesive to secure the gain medium 1-105 in a mount can provide stressrelief for the gain medium 1-105.

The inventors have further recognized and appreciated that mountingstructures that dissipate heat, can adversely affect optical alignmentof a laser cavity. For example, a mount 3-100 for the gain medium 1-105and/or the pump diode module 2-140 can be fastened to the base chassis2-105 and dissipate heat into the base chassis. Since the base chassisis comparatively small for high-power lasers, this heating can causeexpansion and/or warping or other distortions of the base chassis. As aresult, distortion of the base chassis 2-105 can misalign opticalelements of the laser cavity and adversely affect the laser's operationover time. In severe cases, the thermal heating can cause an appreciablydegradation in power and can terminate mode-locking of the laser.

In some embodiments, a mounting structure or component of a mode-lockedlaser that requires significant heat dissipation can be mounted on apartially-thermally-isolated platform 3-610, as depicted in plan view inFIG. 3-6A. The platform can partially thermally isolate the baseplatebody 3-605 from heat dissipated by the high-temperature structure orcomponent mounted on the platform 3-610. Elevation views of theplatform, taken at the cut lines in FIG. 3-6A, are depicted in FIG. 3-6Band FIG. 3-6C. A partially-isolated platform 3-610 can be formed in abaseplate 2-105 by a machining process, according to someimplementations. For example, the baseplate body 3-605 can be part of asolid block of material that is machined to form a housing for acompact, mode-locked laser as described above. One or morethrough-trenches 3-630 can be machined through the baseplate body 3-605to form the partially-isolated platform 3-610. These trenches canpartially separate and thermally isolate the platform 3-610 from thebaseplate 3-605. For example, heat cannot be dissipated as readily fromthe platform into the baseplate. A lower surface of the platform 3-610can be thermally contacted to a thermal-electric cooler (not shown),according to some implementations. In some cases, a lower surface of theplatform can be machined or otherwise formed to have heat-dissipatingfins 3-612.

A plurality of support bridges 3-620 can remain after the machiningprocess that forms the trenches 3-630. The support bridges providemechanical support for the platform 3-610, and reduce thermal conductionfrom the platform 3-610 to the baseplate 3-605. In some embodiments, thebridges can be formed from a different material than the platform. Invarious embodiments, the bridges 3-620 are located centrally, withrespect to the thickness of the platform, between upper and lowersurfaces of the platform 3-610, as depicted in FIG. 3-6B. For example,the bridges 3-620 can be located in a neutral mechanical plane of thebaseplate 3-605 as illustrated in FIG. 3-6B. Locating the bridges 3-620centrally with respect to the thickness of the platform and baseplatecan reduce the amount of out-of-plane thermal-mechanical stress impartedbetween the baseplate body 3-605 and platform 3-610. Reducing the amountof heat dissipated into the baseplate and reducing out-of-plane stresscan reduce warping of the baseplate and undesired relative motion ofother optical components in the laser cavity. In some embodiments, thebridges comprise flexural members that allow the platform to movein-plane relative to the baseplate 2-105, e.g., to accommodatethermo-mechanical stresses induced by the platform. Motion of some lasercomponents (e.g., the gain medium 1-105) may not affect operation of thelaser as much as other components (e.g., cavity mirrors), and thereforecan be tolerated. The partial thermo-mechanical isolation of theplatform 3-610 can improve the stability of the laser, and reduce theneed for adjustments by a skilled operator.

According to some embodiments, one or more platforms 3-610 can be usedto support high temperature components in a mode-locked laser. Forexample, a first platform 3-610 can be used to support a diode pumpsource, and a second platform can be used to support a laser's gainmedium. In some implementations, a third platform can be used to supportfrequency-doubling component (e.g., a non-linear crystal).

Although the laser cavity described above indicates that the gain mediumis a neodymium vanadate crystal, other types of materials can be used toobtain lasing and mode locking at other wavelengths. Correspondingly,different pump sources can be used to provide pump wavelengths suitablefor exciting the gain medium. According to some embodiments, a pumpwavelength λ_(p) for a compact laser module can be between 390 nm andapproximately 1100 nm. A mode-locked lasing wavelength λ₁ for a compactlaser module can be between 750 nm and 1500 nm. In some cases, an outputwavelength λ₂ for a compact laser module can be frequency doubled andcan be between 325 nm and 750 nm. The frequency-doubling element 3-109can be KTP, LBO or BBO in some implementations. In some cases, an outputwavelength λ₂ can be between 500 nm and 700 nm. An output pulse durationat the fundamental wavelength λ₁ or the frequency-doubled wavelength λ₂can be between 1 picosecond and 100 picoseconds, according to someembodiments. In some cases, the output pulse duration can be between 10picoseconds and 30 picoseconds.

As alternative examples, if a green output wavelength is desired, thegain medium may be Nd:YAG, or Nd:YLF, which lase at 1064 nm and 1053 nm,respectively. In some embodiments, Cr:Forsterite may be used as a gainmedium, which can lase at 1280 nm and be frequency doubled to 640 nm (inthe red region of the optical spectrum). In some embodiments, Pr:LiYF₄may be used as the gain medium and lase at 640 nm (in the red) directly,without frequency doubling. The inventors have recognized andappreciated that Nd:YVO₄ may be used as a gain medium to lase at one ortwo wavelengths 1064 nm and/or 1342 nm, which can be doubled to 532 nm(green) and/or 671 nm (red). The inventors have also recognized andappreciated that sum-frequency generation can be performed in anonlinear crystal to obtain additional wavelengths. For example, pulsesat the two lasing wavelengths from Nd:YVO₄ can be mixed in a nonlinearcrystal to produce radiation at approximately 594 nm. Other gain mediainclude, but are not limited to ytterbium-doped YAG (Yb:YAG),ytterbium-doped glass (Yb:glass), erbium-doped YAG (Er:YAG), andtitanium-doped sapphire (Ti:sapphire).

Pump Source and Module

To excite the gain medium 1-105 and initiate mode-locked operation ofthe laser, continuous-wave radiation (indicated by the black dotted linein FIG. 2-1 and FIG. 4-1) from a high-power laser diode can be focusedinto the gain medium using a coupling lens 2-142. The optical power fromthe laser diode can be between 1 Watt and 20 Watts, which are powerlevels associated with significant electrical and optical heatgeneration. Such heat generation, if allowed to dissipate in the basechassis 2-105 could adversely affect stability of the mode-locked lasermodule 1-108. The laser diode can be mounted in a pump module 2-140 thatis mounted in a through-hole 2-145 in the base chassis 2-105 in a waythat reduces heat conduction from the pump module to the base chassisand helps thermally isolate the pump module 2-140 from the base chassis2-105.

An example of a pump module 2-140 is depicted in FIG. 4-1, according tosome embodiments. The pump module can seal the laser diode 4-130 in aclosed housing 4-110, provide heat dissipation for the laser diode, andinclude an adjustable head 4-120 that can align the pump beam to anoptical axis of the mode-locked laser cavity. An example of a laserdiode pump source that can be used in a pump module is laser diode modelFL-FM01-10-808 available from FocusLight Corporation of Xi'an, Shaanxi,China. In some embodiments, the laser diode 4-130 can be mounted in an Fmount or C mount within the pump module 2-140.

According to some embodiments, the pump-module housing 4-110 can attachsecurely to the base chassis 2-105 with screws and/or stand-off posts4-152 that have low thermal conductivity (e.g., stainless steel, nylon,hard plastic). Part of the housing 4-110 can protrude from a back sideof the base chassis 2-105, and part of the housing can extend through athrough hole 2-145 in the base chassis 2-105. The gaps between thepump-module housing 4-110 and base chassis 2-105 and thelow-thermal-conductivity screws or fasteners help to thermally isolatethe pump module 2-140 from the base chassis 2-105 and maintain stabilityof the mode-locked laser 1-110. The adjustable head 4-120 can similarlyattach to the housing 4-110 with screws 4-154 having a low thermalconductivity. The housing can have heat-dissipating fins 4-124 machinedinto the body to aid in heat extraction from the laser diode 4-130. Afan (not shown) can be mounted nearby or mounted to the housing 4-110 tofurther aid in heat removal. According to some embodiments, the laserdiode 4-130 can be mounted on a thermo-electric cooler (TEC) 4-160 thatallows thermal control and temperature stabilization of the laser diode4-130. In some implementations, a PCB 4-170 that includes circuitry forconnecting to and operating the laser diode 4-130 and/or TEC can attachto the housing 4-110 and form a cover that helps seal the laser diode4-130 from exposure to external dust and humidity.

According to some embodiments, the pump module 2-140 can be locatedwithin about 30 mm of an edge of the base chassis 2-105, and thedissipated heat directed toward the edge and away from the base chassisby a fan, for example. The base chassis 2-105 can serve additionally asa wind screen, protecting the laser optics and laser cavity on one sideof the base chassis from air flow or turbulence on the reverse side ofthe plate where heat is removed. In embodiments, the mounting of thepump-module housing 4-110 as described allows it to be located near thegain medium 1-105 of the mode-locked laser, which helps improvestability of the location of the pump beam 4-135 within the gain medium(improving mode-locking stability) and also helps provide a compactmode-locked laser module 1-108.

The pump-module housing 4-110 can also include beam collimating optics,according to some embodiments. A fast-axis collimator (FAC) 4-142 can belocated near or integrated within the laser diode. In some cases, thiscollimater can comprise a cylindrical lens or pair of crossedcylindrical lenses. In some embodiments, the FAC 4-142 can comprise asingle cylindrical lens and can have a short focal length (e.g., lessthan about 5 mm). In some implementations, the FAC 4-142 can comprise alength of optical fiber having a diameter less than about 150 microns,and its focal length can be less than 500 microns. The FAC 4-142 can beconfigured to provide a beam for the laser diode that has approximatelyequal divergence in orthogonal transverse directions between 5 degreesand 15 degrees. In some embodiments, a beam from the laser diode 4-130and FAC 4-142 can have a rectangular or square cross section (e.g.,corresponding to an array of emitters).

The pump-module housing 4-110 can further include a collimating lens4-144 that collimates the laser diode pump beam. According to someembodiments, this lens can be a plano-convex lens with both surfacesanti-reflection coated for the pump wavelength λ_(p). The planar surfaceof the lens can face the diode 4-130. The focal length of thecollimating lens 4-144 can be between 15 mm and 35 mm. The collimatinglens 4-144 can be spaced from the FAC lens 4-142 by a distanceapproximately equal to the back focal length of the collimating lens4-144, according to some implementations.

A collimated, or nearly collimated, beam from the pump-module housing4-110 can be reflected by a dichroic mirror 4-146 that is mounted in theadjustable pump head 4-120. The dichroic mirror 4-146 can include amultilayer coating that reflects the pump wavelength toward the couplinglens 2-142 and gain medium 1-105 (not shown in FIG. 4-1) and transmitsmode-locked pulses at the lasing wavelength from the mode-locked lasercavity. Since the pump head 4-120 is adjustable with adjustment screws4-154, the dichroic mirror can be pitched (rotated about an axisparallel to the X axis shown in the drawing) and rolled (rotated aboutan axis parallel to the Z axis) to adjust the position of the pump beamwithin the gain medium.

According to some implementations, the location of the dichroic mirror4-146 at which the pump beam 4-135 from the laser diode 4-130 isincident on the mirror 4-146 is positioned approximately at the focallength of the coupling lens 2-142. Because of this positioning, angularadjustments to the dichroic mirror 4-146, which deviate the outgoingpump beam 4-135, result in primarily parallel beam-path displacements ofthe pump beam through the gain medium 1-105. This can be understoodsince rays going from a focal point at the dichroic mirror 4-146 throughthe coupling lens 2-142 will emerge from the coupling lens as parallelrays. Accordingly, the dichroic mirror 4-146 can be adjusted toprimarily translate the pump beam in X and Y directions (referring toFIG. 4-1 and FIG. 2-1) in the gain medium with minimal change to theangle of the pump beam through the gain medium 1-105. In view of thiseffect, pitch adjustments to the dichroic mirror 4-146 result inpump-beam displacements in the Y direction at the gain medium and rolladjustments to the dichroic mirror provide X-directed (and someY-directed) pump-beam displacements in the gain medium 1-105. A changein angle of the pump beam 4-135 through the gain medium 1-105 can beundesirable because it can reduce the overlap volume of the pump beam4-135 with the mode-locked laser beam in the gain medium 1-105.

To simplify assembly and reduce the number of adjustable screws on thepump head 4-120, the adjustable head can be attached to the housing4-110 with a three-point contact, adjustable, kinematic mounting scheme.The head 4-120 can be drawn or forced toward the housing 4-110 using atleast one resilient spring 4-157 (as depicted in FIG. 4-2A and FIG.4-2B). One of the three-point contacts can be a ball-and-cone thatallows all rotational degrees of freedom of the head. For example, aball-shaped contact feature 4-155 (e.g., a ball bearing) may be locatedon a side of the head 4-120 near the pump-housing module 4-110. Theball-shaped contact feature 4-155 may be received by a cone-shapedrecess. The remaining two points of contact can be adjustment screws4-154 a, 4-154 b. One of these screws 4-154 a can have a ball-shaped endthat is received in a groove to restrict yaw motion of the head. Theother screw 4-154 b can have a ball-shaped end that is received on aflat surface. In some implementations, after the adjustment screws 4-154a, 4-154 b have been set to align the pump beam 4-135 through the gainmedium 1-105, at least one counter-force screw 4-158 can be tightened tolock the adjustable head in place.

In some implementations, a focal length of the coupling lens 2-142 canbe between about 20 mm and about 30 mm. The lens can be plano-convex insome embodiments, though a double convex lens can be used in some cases.The coupling lens can have anti-reflection coatings on both sides forthe pump wavelength and mode-locked laser wavelength. Additionally, thecoupling lens 2-142 can be oriented at an angle between 1 degree and 4degrees with respect to the optical axis 2-111 of the mode-locked laser(referring to FIG. 2-1) to avoid reflections from the faces that wouldreturn into the laser cavity and the laser diode. In someimplementations, the gain medium 1-105 is located approximately aback-focal length away from the coupling lens 2-142. Unabsorbed pumpradiation can pass through a laser-cavity folding mirror 2-115 and beabsorbed in a beam dump and/or detected by a photodetector 2-116,according to some embodiments.

In some implementations, the FAC 4-142, collimating lens 4-144, andcoupling lens 2-142 are arranged to provide approximately 1-to-1 imagingof an output from the laser diode 4-130 into the gain medium 1-105. Theimage of the laser diode's output (e.g., emitter array) in the gainmedium should approximately match the mode-locked laser's intracavitybeam waist size in the gain medium. The mode-locked laser's beam waistsize in the gain medium can be determined predominantly by a focallength of curved mirror 2-117, its distance from the output coupler1-111, and a distance of the gain medium 1-105 from the output coupler.For the configurations described above and with 1-to-1 imaging, theimaged emitter size of the laser diode in the gain medium should bebetween 100 microns and 150 microns. The inventors have observed thatemitter sizes between 90 microns and 220 microns provide mode-lockedlasing, though the small emitter size is more susceptible to rapiddegradation and the larger emitter size can cause the mode-locked laserto lase in higher order spatial modes. Additionally, the polarization ofradiation should be well matched to the intended lasing polarization ofthe mode-locked laser 1-110. In this regard, the pump module 2-140and/or the mode-locked laser module 1-108 can include one or moreoptical components (e.g., quarter-wave plate, half-wave plate) that canbe rotated or adjusted to control the state of polarization of the pumpradiation on the gain medium 1-105. The polarization can be controlledto increase lasing efficiency and output power from the mode-lockedlaser 1-110.

Other excitation sources can be used to pump the gain medium 1-105 inother embodiments, and the invention is not limited to laser diodes. Insome embodiments, a fiber or fiber-coupled laser (not shown) can be usedto pump the gain medium 1-105 of the mode-locked laser 1-110. A fiberlaser can comprise an active optical fiber as part of the fiber-lasercavity that is pumped by one or more laser diodes. A fiber-coupled lasercan comprise one or more laser diodes having their outputs coupled intoan optical fiber. An output beam from a fiber carrying optical energyfrom the fiber laser or fiber-coupled laser can be directed to andfocused into the gain medium using the same or similar optics that areused for a laser diode pump source. An optical beam from a fiber canhave a more circular, homogenous, and/or Gaussian (or top-hat-shaped)spatial profile than a beam directly from a high-power laser diode pumpsource. A fiber laser pump source may or may not be mounted on a fixtureother than base chassis 2-105 in some embodiments, and an end of thefiber carrying pump energy can be attached to a mount on the mode-lockedlaser module 1-108 that is located on the same side or opposite side ofthe base chassis as the gain medium 1-105.

Cavity Alignment

As may be appreciated, alignment of the mode-locked laser-cavity opticscan be difficult because of the high number of mirrors and opticalcomponents in the laser cavity. In some embodiments and referring againto FIG. 2-1, a mode-locked laser can include mounting features 2-110(e.g., screw holes and/or registration features) located along theoptical axis of the laser cavity between the gain medium 1-105 andsecond curved mirror 2-127. The mounting features 2-110 can beconfigured to receive an optical mount in which a second output coupler(not shown in FIG. 2-1) can be mounted. When the optical mount andsecond output coupler are in place, the laser can be aligned to lase incontinuous-wave mode with a shortened laser cavity. The second outputcoupler can transmit a small amount of power (e.g., between 2% and 20%),and provide a laser beam that can be used to align optical components ofthe laser between the inserted optical mount and the SAM 1-119. Oncethese remaining components are aligned, the inserted optical mount canbe removed, so that the laser 1-110 can be tuned to operate in pulsedmode with the full cavity length.

The inventors have discovered that a second output coupler (not shown inFIG. 2-1) for short cavity alignment can be mounted near the gain medium1-105 and before the turning mirror 2-115. According to someembodiments, thermal lensing in the gain medium, when pumped at opticalpowers that enable mode-locked operation of the mode-locked laser 1-108,supports lasing in the shortened cavity and provides a stable lasingcavity without the need for an additional lensing element in theshortened cavity, even though the length of the shortened cavity is lessthan half the length of the mode-locked laser cavity. In some cases, thelength of the shortened cavity can be less than one-quarter or evenone-eighth the length of the mode-locked laser cavity. Accordingly, asecond output coupler placed near the gain medium 1-105 can enable easyand rapid alignment of all optical elements from the turning mirror2-115 to the SAM 1-119. In a configuration with an output couplermounted before the turning mirror 2-115 and no other lensing element inthe shortened laser cavity, it can be helpful to have thermal lensing inthe gain medium of at least 2 diopters to obtain lasing and makealignment of the shortened cavity easier, though in some casescontinuous wave lasing can occur without thermal lensing (0 diopters).

Frequency Doubling

Referring again to FIG. 2-1, an output of a mode-locked laser 1-110 canbe focused through a lens 2-164 into a frequency-doubling crystal 2-170to halve the optical wavelength (or double the optical frequency) of theoutput pulses. For example, the mode-locked laser 1-110 can producepulses with a characteristic wavelength of about 1064 nm, and thefrequency-doubling crystal 2-170 can convert the wavelength to about 532nm. The frequency-doubled output can be provided to a bio-optoelectronicchip 1-140 and used there to excite fluorophores having differentemission characteristics. Components for frequency doubling and controlof the frequency-doubled power can be mounted within the compactmode-locked laser module 1-108, according to some embodiments.

The lens 2-164 can have a focal length between 15 mm and 30 mm, andinclude antireflection coatings on both surfaces to minimize reflectionsof the lasing wavelength. The lens can produce a beam waist for themode-locked pulses between 15 microns and 35 microns in thefrequency-doubling crystal.

The frequency-doubling crystal 2-170 can be a potassium titanylphosphate (KTP), type II crystal. The crystal length can be between 3 mmand 7 mm. According to some embodiments, the frequency-doubling crystal2-170 is a high grey track resistant (HGTR) crystal. The inventorsobserved that flux grown crystals can degrade for high average powers atgreen wavelengths. For the HGTR crystals, the cut angles can be between24 degrees and 25 degrees for phi and between 89 degrees and 91 degreesfor theta. Both facets of the crystal can be coated with anti-reflectioncoatings for the lasing wavelength and the doubled wavelength. Accordingto some implementations, the frequency-doubling crystal 2-170 is mountedagainst a self-aligning surface formed in the base chassis 2-105. Acollimating lens (not shown in FIG. 2-1) can be placed after thefrequency-doubling crystal to collimate the frequency-doubled radiationfrom the crystal.

In some embodiments, a half-wave plate 2-160 can be mounted in arotatable mount with its rotation angle controlled by an actuator 2-162.The half-wave plate can be located in the output optical path of themode-locked laser before the frequency-doubling crystal 2-170. Accordingto some embodiments, an actuator 2-162 can comprise a stepper motor, apiezoelectric motor, a galvanometer having precision bearings andconfigured to rotate an optical component, a DC motor, or any othersuitable actuation mechanism. Rotating the half-wave plate 2-160 canalter the polarization of the laser's output pulses and change thesecond-harmonic conversion efficiency in the frequency-doubling crystal2-170. Control of the half-wave plate can then be used to control anamount of power at the frequency-doubled wavelength that is delivered tothe bio-optoelectronic chip 1-140. By rotating the half-wave plate 2-160(or the frequency-doubling crystal 2-170), the optical power at thefrequency-doubled wavelength λ₂ can be varied precisely by small amountsover a large range (e.g., over an order of magnitude or more), withoutaffecting the operation of the mode-locked laser at the fundamentalwavelength λ₁. That is, the power at the frequency-doubled wavelengthcan be altered without affecting the mode-locking stability, thermaldissipation, and other characteristics of the mode-locked laser 1-110.In some embodiments, other adjustments can be used additionally oralternatively to control frequency-doubled power without affecting thefundamental laser operation. For example, an incident angle of thepulsed-laser beam on the frequency-doubling crystal 2-170 and/ordistance between the lens 2-164 and frequency-doubling crystal can becontrolled in an automated manner to alter and/or maximize thefrequency-doubling efficiency.

In some embodiments, the frequency-doubled output pulses can be directedby a turning mirror 2-180 and/or to a beam shaping and steering module.The turning mirror 2-180 can be dichroic, such that it transmits opticalradiation which has not been down-converted by the frequency-doublingcrystal 2-170 to a beam dump (not shown). In some implementations, theturning mirror 2-180 can transmit a small amount of thefrequency-doubled output to a photodiode 2-182. A wavelength selectivefilter can be placed in front of the photodiode 2-182 to block orreflect the fundamental wavelength. An output from the photodiode 2-182can be provided to the PCB 2-190 where the signal can be processed toevaluate mode-locking stability and/or produce a control signal forrotating the half-wave plate 2-160 to maintain a stable output power. Insome implementations, the photodiode 2-182 can be mounted on the PCB2-190 and the frequency-doubled output can be reflected, scattered,coupled via an optical fiber, or otherwise directed to the photodiodethrough a hole and/or window in the base chassis 2-105.

In some implementations, a beam shaping and steering module as describedin a separate U.S. patent application No. 62,435,679, filed Dec. 16,2016 and titled “Compact Beam Shaping and Steering Assembly” can beassembled on the baseplate or mounted adjacent to the base chassis2-105. An output beam from the laser module can be provided to the beamshaping and steering assembly to adapt the output beam at thefundamental wavelength or frequency-doubled wavelength for use in ananalytic system 1-160.

Clock Generation and System Control

Referring again to FIG. 1-1, regardless of the method and apparatus thatis used to produce short or ultrashort-pulses, a portable analyticinstrument 1-100 can include circuitry configured to synchronize atleast some electronic operations (e.g., data acquisition and signalprocessing) of an analytic system 1-160 with the repetition rate ofoptical pulses 1-122 from the mode-locked laser 1-110. For example, whenevaluating fluorescent lifetime in a bio-optoelectronic chip 1-140, itis beneficial to know the time of excitation of a sample accurately, sothat timing of emission events can be correctly recorded. According tosome embodiments, a timing signal can be derived from the optical pulsesproduced by the mode-locked laser, and the derived timing signal can beused to trigger instrument electronics.

The inventors have recognized and appreciated that coordination ofoperation of the mode-locked laser 1-110 (e.g., to deliver excitationoptical pulses to reaction chambers 1-330), signal-acquisitionelectronics (e.g., operation of time-binning photodetectors 1-322), anddata read-out from the bio-optoelectronic chip 1-140 poses technicalchallenges. For example, in order for the time-binned signals collectedat the reaction chambers to be accurate representations of fluorescentdecay characteristics, each of the time-binning photodetector 1-322 mustbe triggered with precise timing after the arrival of each excitationoptical pulse at the reaction chambers. Additionally, data must be readfrom the bio-optoelectronic chip 1-140 in approximate synchronicity withdata acquisition at the reaction chambers to avoid data overruns andmissed data. Missed data could be detrimental in some cases, e.g.,causing a misrecognition of a gene sequence. The inventors haverecognized and appreciated that system timing is further complicated bythe natural operating characteristics of passively mode-locked lasers,e.g., prone to fluctuations in pulse amplitude, fluctuations inpulse-to-pulse interval T, and occasional pulse drop-outs.

FIG. 5-1 depicts a system in which a timer 5-120 provides asynchronizing signal to the analytic system 1-160. In some embodiments,the timer 5-120 can produce a clock signal that is synchronized tooptical pulses produced by the pulsed optical source 1-110, and providethe clock signal to the analytic system 1-160. In FIG. 5-1, the opticalpulses 1-120 are depicted spatially as being separated by a distance D.This separation distance corresponds to the time T between pulsesaccording to the relation T=D/c where c is the speed of light. Inpractice, the time T between pulses can be determined with a photodiodeand oscilloscope. According to some embodiments, T=1/f_(sync)N where Nis an integer greater than or equal to 1 and f_(sync) represents thefrequency of a generated clock signal. In some implementations, T=N/fsync where N is an integer greater than or equal to 1.

According to some embodiments, the timer 5-120 can receive an analog ordigitized signal from a photodiode that detects optical pulses from thepulse source 1-110. The photodiode 2-154 can be mounted on the basechassis 2-105 and can be a high-speed InGaAs photodiode. The timer 5-120can use any suitable method to form or trigger a synchronizing signalfrom the received analog or digitized signal. For example, the timer canuse a Schmitt trigger or comparator to form a train of digital pulsesfrom detected optical pulses. In some implementations, the timer 5-120can further use a delay-locked loop or phase-locked loop to synchronizea stable clock signal from a stable electronic clock source to a trainof digital pulses produced from the detected optical pulses. The trainof digital pulses or the locked stable clock signal can be provided tothe analytic system 1-160 to synchronize electronics on the instrumentwith the optical pulses.

The inventors have conceived and developed clock-generation circuitrythat can be used to generate a clock signal and drive data-acquisitionelectronics in a portable instrument 1-100. An example ofclock-generation circuitry 5-200 is depicted in FIG. 5-2. The clockgeneration circuitry can be included on a PCB 2-190 mounted on the basechassis 2-105. According to some embodiments, clock-generation circuitrycan include stages of pulse detection, signal amplification withautomatic gain control, clock digitization, and clock phase locking.

A pulse-detection stage can comprise a high-speed photodiode 5-210 thatis reversed biased and connected between a biasing potential and areference potential (e.g., a ground potential), according to someembodiments. A reverse bias on the photodiode can be any suitable value,and can be fixed using fixed-value resistors or can be adjustable. Insome cases, a capacitor C can be connected between a cathode of thephotodiode 5-210 and a reference potential. A signal from the anode ofthe photodiode can be provided to an amplification stage. In someembodiments, the pulse detection stage can be configured to detectoptical pulses having an average power level between about 100microwatts and about 25 milliwatts. The pulse-detection stage of theclock-generation circuitry 5-200 can be mounted on or near themode-locked laser 1-110, and arranged to detect optical pulses from thelaser.

An amplification stage can comprise one or more analog amplifiers 5-220that can include variable gain adjustments or adjustable attenuation, sothat pulse output levels from the analog gain amplifiers can be setwithin a predetermined range. An amplification stage of theclock-generation circuitry 5-200 can further include an automatic gaincontrol amplifier 5-240. In some cases, analog filtering circuitry 5-230can be connected to an output of the analog amplifiers 5-220 (e.g., toremove high-frequency (e.g., greater than about 500 MHz) and/orlow-frequency noise (e.g., less than about 100 Hz)). The filtered orunfiltered output from the one or more analog gain amplifiers 5-220 canbe provided to an automatic gain control amplifier 5-240, according tosome embodiments.

According to some embodiments, a final output signal from the one ormore analog amplifiers can be positive-going. The inventors haverecognized and appreciated that a subsequent automatic gain-control(AGC) amplifier operates more reliably when it input pulses to positivevoltage rather than negative voltage. The automatic gain controlamplifier can vary its internal gain to compensate for amplitudefluctuations in the received electronic pulse train. The output pulsetrain from the automatic gain control amplifier 5-240 can haveapproximately constant amplitude, as depicted in the drawing, whereasthe input to the automatic gain control amplifier 5-240 can havefluctuations in the pulse-to-pulse amplitudes. An example automatic gaincontrol amplifier is model AD8368 available from Analog Devices, Inc. ofNorwood, Mass.

In a clock digitization stage, an output from the automatic gain controlamplifier can be provided to a comparator 5-250 to produce a digitalpulse train, according to some implementations. For example, the pulsetrain from the AGC can be provided to a first input of the comparator5-250, and a reference potential (which can be user-settable in someembodiments) can be connected to a second input of the comparator. Thereference potential can establish the trigger point for the rising edgeof each produced digital pulse.

As may be appreciated, fluctuations in optical pulse amplitude wouldlead to fluctuations in amplitudes of the electronic pulses before theAGC amplifier 5-240. Without the AGC amplifier, these amplitudefluctuations would lead to timing jitter in the rising edges of pulsesin the digitized pulse train from the comparator 5-250. By leveling thepulse amplitudes with the AGC amplifier, pulse jitter after thecomparator is reduced significantly. For example, timing jitter can bereduced to less than about 50 picoseconds with the AGC amplifier. Insome implementations, an output from the comparator can be provided tologic circuitry 5-270 which is configured to change the duty cycle ofthe digitized pulse train to approximately 50%.

A phase-locking stage of the clock-generation circuitry 5-200 cancomprise a phase-locked loop (PLL) circuit 5-280 that is used to produceone or more stable output clock signals for timing and synchronizinginstrument operations. According to some embodiments, an output from theclock digitization stage can be provided to a first input (e.g., afeedback input) of a PLL circuit 5-280, and a signal from an electronicor electro-mechanical oscillator 5-260 can be provided to a second input(e.g., a reference input) to the PLL. An electronic orelectro-mechanical oscillator can be highly stable against mechanicalperturbations and against temperature variations in some cases.According to some embodiments, a phase of the stable clock signal fromthe electronic or electro-mechanical oscillator 5-260 is locked, by thePLL, to a phase of the digitized clock signal derived from themode-locked laser, which can be less stable. In this manner, theelectronic or electro-mechanical oscillator 5-260 can ride throughshort-term instabilities (e.g., pulse jitter, pulse drop outs) of themode-locked laser 1-110, and yet be approximately synchronized to theoptical pulse train. The phase-locked loop circuit 5-280 can beconfigured to produce one or more stable output clock signals that arederived from the phase-locked signal from the electro orelectro-mechanical oscillator 5-260. An example circuit that can be usedto implement the PLL is IC chip Si5338, which is available from SiliconLaboratories Inc. of Austin, Tex.

According to some embodiments, one or more clock signals output from thePLL circuit 5-280 can be provided to the bio-optoelectronic chip 1-140to time data-acquisition electronics on the chip. In some cases, the PLLcircuit 5-280 can include phase adjustment circuitry 5-282, 5-284 on itsclock outputs, or separate phase adjustment circuits can be connected toclock outputs of the phase-locked loop. In some implementations, thebio-optoelectronic chip 1-140 can provide a pulse-arrival signal 1-142from one or more photodetectors on the chip that indicate the arrival ofoptical excitation pulses from the mode-locked laser 1-110. Thepulse-arrival signal can be evaluated and used to set the phase orphases of clock signals provided to the bio-optoelectronic chip 1-140.In some embodiments, the pulse-arrival signal can be provided back tothe phased-locked loop circuit 5-280 and processed to automaticallyadjust the phase of the clock signal(s) provided to the chip, so that atrigger edge of a clock signal provided to drive data-acquisition on thebio-optoelectronic chip 1-140 (e.g., timing of signal acquisition by thetime-binning photodetectors 1-322) is adjusted to occur at apredetermined time after the arrival of an optical excitation pulse inthe reaction chambers.

According to some embodiments, a clock signal from the PLL circuit 5-280can also be provided to one or more field-programmable gate arrays(FPGAs) 5-290 included in the instrument 1-100. The FPGAs can be usedfor various functions on the instrument, such as driving data read outfrom the bio-optoelectronic chip 1-140, data processing, datatransmission, data storage, etc.

The inventors have recognized and appreciated that there can be aninterplay between the loop bandwidth of the AGC amplifier 5-240 and theloop bandwidth of the phase-locked loop 5-290. For example, if the loopbandwidth of the phase-locked loop is too high, the PLL can respond tojitter introduced by the AGC amplifier and comparator in the digitizedpulse train, and not accurately track the optical pulse timing. On theother hand, if either or both of the AGC and PLL loop bandwidths are toolow, the resulting clock signals output from the PLL will not accuratelytrack the optical pulse timing. The inventors have found that anintegration time constant associated with the loop bandwidth of the PLL5-290 should be between about 30 pulses and about 80 pulses of theoptical pulse train from the mode-locked laser 1-110. Additionally, anintegration time constant associated with the loop bandwidth of the AGCamplifier 5-240 should not exceed by more than about 20% the integrationtime constant for the PLL.

In some implementations, one or more signals from the amplificationstage can be used for additional purposes in the instrument 1-100. Forexample, an analog signal 5-232 can be split off prior to the AGCamplifier 5-240 and used to monitor the quality of mode locking in themode-locked laser 1-110. For example, the analog signal 5-232 can beanalyzed electronically in the frequency and/or time domain to detectcharacteristics that are indicative of the onset of Q-switching by themode-locked laser. If the characteristics (and onset of Q-switching) aredetected, the system can automatically make adjustments to optics withinthe mode-locked laser (e.g., cavity-alignment optics) to avoidQ-switching, or the system can indicate an error and/or shut down themode-locked laser.

In some embodiments, an AGC amplifier can provide an output signal 5-242(analog or digital) that is representative of real-time gain adjustmentsthat are needed to level the amplitudes of the output pulses. Theinventors have recognized and appreciated that this output signal 5-242can be used to evaluate mode-locking quality of the mode-locked laser.For example, its spectrum can be analyzed to detect the onset ofQ-switching.

Although clock generation and synchronization has been described usingan automatic gain control amplifier and a phase-locked loop, alternativeapparatus can be used in other embodiments for which a larger amount ofclock jitter (e.g., up to about 300 ps) can be tolerated. In someimplementations, an amplifier in the pulse amplification stage can bedriven into saturation to provide a rising edge trigger signal. Atrigger point for a clock can be set at some value on the rising edge.Because the amplifier saturates, variations in pulse amplitude have lessof an effect on the trigger timing than for a non-saturated amplifier.The rising edge can be used to toggle a flip-flop clocking circuit, suchas those implemented in field-programmable gate arrays (FPGAs). Thefalling edge from the saturated amplifier returning back to zero canhave appreciably more timing variability, depending on when the outputof the amplifier is released from saturation. However, the falling edgeis not detected by the flip-flop clocking circuit and has no effect onthe clocking.

Many FPGAs include digital delay-lock loops (DLL) which can be usedinstead of a PLL to lock a stable oscillator to the laser-generatedclocking signal from the flip flop. In some embodiments, the receivingflip-flop divides the clocking rate from the optical pulse train by two,which can provide a 50% duty-cycle clock signal to the DLL at one-halfthe pulse repetition rate. The DLL can be configured to generate afrequency-doubled clock to be synchronized with the optical pulse train.Additional synchronized, higher-frequency clocks can also be generatedby the DLL and FPGA.

An example of system circuitry for system control is depicted in FIG.5-3, according to some embodiments. A pump-module control circuit 5-300can be assembled on a PCB and mounted to the compact mode-locked lasermodule 1-108 (e.g., mounted on a back side of the module 1-108 shown inFIG. 2-1). The pump-module control circuit 5-300 can interface with asystem board 5-320 and a clock-generation and laser-sensing circuit5-350 (e.g., PCB 2-190) that is mounted on the laser module 1-108. Insome implementations, the pump-module control circuit 5-300 andclock-generation and laser-sensing circuit 5-350 can be assembled on asame PCB. In other implementations, the pump-module control circuit5-300, clock-generation and laser-sensing circuit 5-350, and systemcontrol circuitry can be assembled on a same PCB, so that a separatesystem board 5-320 is not used.

The system board 5-320 can include a central processor (e.g., amicrocontroller or microprocessor) that coordinates operation of thesystem in which the laser module 1-108 is mounted. The system board5-320 can further include power distribution circuitry and data handlingcircuitry (e.g., memory, transceiver, network interface board, etc.).

In some embodiments, the pump-module control circuit 5-300 can include acurrent source 5-332 configured to supply current to the laser diode4-130 that is used to pump the gain medium 1-105. The current source5-332 can be controlled via the system board 5-320, according to someembodiments. The pump-module control circuit 5-300 can further includetemperature sensing circuitry 5-341 that can connect to a temperaturesensor or thermistor (not shown) on the laser diode 4-130. Output fromthe temperature sensing circuitry 5-341 can be provided to temperaturecontrolling circuitry 5-343, which can drive a TEC 4-160 on which thelaser diode 4-130 is mounted. The temperature controller can receivecontrol signals from the system board 5-320 for adjusting and/orstabilizing a temperature of the laser diode 4-130, according to someembodiments.

In some implementations, the pump-module control circuit 5-300 caninclude one or more actuator control circuits (two shown) 5-351, 5-352.The actuator control circuits can receive control signals from thesystem board 5-320 to operate one or more actuators located on themode-locked laser module 1-108. For example, a first actuator controlcircuit 5-351 can be configured to operate a first actuator 2-162 thatrotates a laser window 2-128 in the laser cavity of the mode-lockedlaser 1-110. Operation of the first actuator can adjust cavity alignmentand be used to improve mode locking of the laser 1-110. A secondactuator control circuit 5-352 can be configured to operate a secondactuator 2-162 that rotates a half-wave plate 2-160 on the laser module1-108, for example. Rotation of the half-wave plate 2-160 can be used tocontrol an amount of laser power converted to a frequency-doubledwavelength, for example.

According to some embodiments, control signals for the actuator circuits5-351, 5-352 can be computed on the system board 5-320 based uponoutputs from the clock-generation and laser-sensing circuit 5-350.Outputs from the clock-generation and laser-sensing circuit 5-350 can beproduced by a fundamental sensor circuit 5-311 (which can include orconnect to a photodiode 2-154 configured to sense a fundamentalwavelength λ₁ from the laser 1-110), a frequency-doubled sensor circuit5-312 (which can include or connect to a photodiode 2-182 configured tosense a frequency-doubled wavelength λ₂ produced from the laser's outputpulses), and a diode pump sensor circuit 5-313 (which can include orconnect to a photodiode 2-116 configured to sense a pump wavelengthλ_(p) used to excite the gain medium 1-105 in the laser 1-110).Accordingly, feedback control of the mode-locked laser 1-110 andfrequency-doubled output power can be implemented by sensing laseroperational and output parameters and applying signals via the actuatorcircuits 5-351, 5-352 that correct or improve operation of themode-locked laser module 1-108. It will be appreciated that someembodiments can include additional sensor circuits and/or additionalactuator control circuits for controlling the same and/or additionalcomponents on the compact mode-locked laser module 1-108.

Embodiments of the described technology include the followingconfigurations and methods.

(1) A mode-locked laser module comprising a base chassis; a mode-lockedlaser having a laser cavity assembled on the base chassis; and a gainmedium located in the laser cavity that exhibits a positive thermallensing value between one diopter and 15 diopters when the mode-lockedlaser is producing optical pulses.

(2) The mode-locked laser module of configuration (1), furthercomprising a laser diode arranged to excite the gain medium with a pumpbeam, wherein absorption of the pump beam in the gain medium causes thethermal lensing.

(3) The mode-locked laser module of configuration (1) or (2), whereinthe gain medium comprises a solid state crystal that is disposed in amount and has no active cooling.

(4) The mode-locked laser module of configuration (2) or (3), whereinthe mode-locked laser produces optical pulses stably without mechanicaladjustments to the laser cavity for thermal lensing values varied over arange from 8 diopters to 12 diopters due to changes in optical power ofthe pump beam.

(5) The mode-locked laser module of any of configurations (1) through(4), wherein the mode-locked laser produces optical pulses stably forthermal lensing values varied over a range from one diopter to 15diopters due to changes in optical power of the pump beam.

(6) The mode-locked laser module of configuration (5), wherein thechanges in the optical power of the pump beam are between 2 Watts and 10Watts and an average output optical power from the mode-locked lasermodule is between 350 milliwatts and 3.5 watts.

(7) The mode-locked laser module of any of configurations (1) through(6), wherein a pulse repetition rate of the optical pulses is between 50MHz and 200 MHz and a maximum edge length of the base chassis is notmore than 350 mm.

(8) The mode-locked laser module of any of configurations (1) through(7), wherein a pulse repetition rate of the optical pulses is between 50MHz and 200 MHz and wherein the module has a slab form with a maximumedge length measuring not more than 350 mm and a thickness measuring notmore than 40 mm and a weight of the module is no more than 2 kilograms.

(9) The mode-locked laser module of any of configurations (1) through(8), wherein a pulse repetition rate of the optical pulses is between 50MHz and 200 MHz and wherein a maximum volume occupied by the mode-lockedlaser module is not more than 0.1 cubic feet.

(10) The mode-locked laser module of any of configurations (1) through(9), wherein a full-width-half-maximum pulse width of the optical pulsesis between 9 picoseconds and 38 picoseconds.

(11) The mode-locked laser module of any of configurations (1) through(10), wherein the gain crystal comprises neodymium vanadate (Nd3+:YVO4).

(12) The mode-locked laser module of any of configurations (1) through(11), further comprising a diagonal rib extending diagonally across thechassis that increases torsional stiffness of the chassis, wherein anintracavity beam of the laser cavity passes through multiple openings inthe diagonal rib.

(13) The mode-locked laser module of any of configurations (1) through(12), further comprising a saturable absorber mirror mounted on a plateat an end of the laser cavity, wherein the plate is configured to beadjusted with only two degrees of freedom which do not include angleadjustments with respect to an optical axis of an intracavity beam ofthe laser cavity that is incident on the saturable absorber mirror.

(14) The mode-locked laser module of configuration (13), wherein theplate comprises a printed circuit board having a metal coating or theplate comprises a plate of metal.

(15) The mode-locked laser module of configuration (13) or (14), whereina first beam waist of the intracavity beam within the gain medium isbetween 100 microns and 150 microns and a second beam waist of theintracavity beam at the saturable absorber is between 75 microns and 125microns.

(16) The mode-locked laser module of any of configurations (13) through(15), further comprising a first focusing optic located within the lasercavity; and a laser window or optical flat located within the lasercavity, wherein the first focusing optic and laser window or opticalflat are arranged to adjust an incident angle of the intracavity beam onthe saturable absorber mirror by rotating the laser window or opticalflat.

(17) The mode-locked laser module of any of configurations (13) through(16), further comprising a cavity length extending region located withinthe laser cavity between the gain medium and the saturable absorbermirror, wherein the cavity length extending region folds the intracavitybeam at least four times.

(18) The mode-locked laser module of configuration (17), wherein thecavity length extending region comprises a first reflector; and a secondfocusing reflector located between the saturable absorber mirror and thegain medium, wherein the first reflector and the second focusingreflector fold the intracavity beam three times on successivereflections.

(19) The mode-locked laser module of configuration (17) or (18), whereinthe cavity length extending region comprises a first reflector thatfolds the intracavity beam multiple times.

(20) The mode-locked laser module of any of configurations (1) through(15), further comprising an output coupler located at a first end of thelaser cavity; a saturable absorber mirror located at a second end of thelaser cavity; a first focusing optic located within the laser cavitybetween the gain medium and the saturable absorber mirror; and a secondfocusing optic located within the laser cavity between the firstfocusing optic and the saturable absorber mirror.

(21) The mode-locked laser module of configuration (20), wherein anintracavity beam between the first focusing optic and the secondfocusing optic is essentially collimated.

(22) The mode-locked laser module of configuration (20) or (21), whereina focal length of the first focusing optic is between 240 mm and 260 mmand a focal length of the second focusing optic is between 240 mm and260 mm.

(23) The mode-locked laser module of any of configurations (20) through(22), wherein the output coupler is located between 280 mm and 300 mmfrom the first focusing optic and the gain medium is located between 4mm and 8 mm from the output coupler.

(24) The mode-locked laser module of any of configurations (1) through(23), further comprising only one mirror located within the laser cavitythat provides angular adjustment of the one mirror while the mode-lockedlaser is operating.

(25) The mode-locked laser module of any of configurations (1) through(24), further comprising a frequency-doubling crystal mounted on thechassis and arranged to double a frequency of an output beam from thelaser cavity.

(26) A mode-locked laser module comprising a base chassis; a mode-lockedlaser having a laser cavity assembled on the base chassis; an outputcoupler mounted on a first mount at a first end of the laser cavity,wherein the first mount provides no angular adjustment of the outputcoupler with respect to an optical axis of an intracavity beam that isincident on the output coupler; a saturable absorber mirror mounted on asecond mount at a second end of the laser cavity, wherein the secondmount provides no angular adjustment of the saturable absorber mirrorwith respect to the optical axis of the intracavity beam that isincident on the saturable absorber mirror; and a gain medium locatedbetween the mode-locked laser and the output coupler.

Configuration (26) can include one or more aspects and features from anyof configurations (2) through (25).

(27) A mode-locked laser module comprising a base chassis; an outputcoupler and a first focusing optic mounted on the base chassis; asaturable absorber mirror and second focusing optic mounted on the basechassis, wherein the output coupler and saturable absorber mirrorcomprise end mirrors of a laser cavity for the mode-locked laser; a gainmedium located along an optical axis of an intracavity beam within thelaser cavity; and a cavity length extending region comprising tworeflectors located between the output coupler and the saturable absorbermirror, wherein the two reflectors fold the intracavity beam more thantwo times.

Configuration (27) can include one or more aspects and features from anyof configurations (2) through (25).

(28) A mode-locked laser module comprising a base chassis; a mode-lockedlaser having a first laser cavity configured to operate at a pulserepetition rate between 50 MHz and 200 MHz, wherein the mode-lockedlaser is assembled on the base chassis; a first end mirror of the firstlaser cavity located at a first end of the first laser cavity; a secondend mirror of the first laser cavity located at a second end of thefirst laser cavity; and a gain medium located within the first lasercavity, wherein the gain medium is configured to exhibit thermal lensingwhen pumped at an operating power for the first laser cavity, whereinthe thermal lensing supports lasing in a second laser cavity formedwithin the first laser cavity that is less than one-half the length ofthe first laser cavity and that includes the first end mirror and athird end mirror that is installed on the base chassis in the firstlaser cavity.

Configuration (28) can include one or more aspects and features from anyof configurations (2) through (25).

(29) A method of operating a mode-locked laser, the method comprisingacts of pumping a gain medium of a laser cavity with an optical pumpbeam, such that the gain medium exhibits thermal lensing having a rangeof diopter values between 8 diopters and 12 diopters; reflecting anintracavity beam from and output coupler at a first end of the lasercavity and a saturable absorber mirror at a second end of the lasercavity; and producing an output of stable optical pulses over the rangeof diopter values.

(30) The method of (29), further comprising pumping the gain medium ofthe laser cavity, such that the gain medium exhibits thermal lensinghaving a range of diopter values between one diopter and 15 diopters.

(31) The method of (29) or (30), further comprising adjusting an amountof the thermal lensing by tuning a wavelength of the optical pump beam.

(32) The method of any of (29) through (31), further comprisingreflecting the intracavity beam from a first focusing reflector and asecond focusing reflector located between the gain medium and thesaturable absorber mirror.

(33) The method of (32), further comprising adjusting an incident angleof the intracavity beam on the saturable absorber mirror withoutadjusting an orientation angle of the saturable absorber mirror withrespect to a chassis supporting the gain medium, the output coupler, andthe saturable absorber mirror.

(34) The method of any one of (29) through (33), further comprisingreflecting the intracavity beam from a plurality of mirrors locatedbetween the gain medium and the saturable absorber mirror to extend alength of the laser cavity.

(35) The method of (34), further comprising reflecting the intracavitybeam between two mirrors of the plurality of mirrors more than two timeson immediately successive reflections.

(36) The method of (34) or (35), further comprising applying an apertureto the intracavity beam to suppress higher-order modes.

(37) The method of any one of (29) through (36), wherein the opticalpulses have a pulse repetition rate between 50 MHz and 200 MHz and achassis on which the output coupler and saturable absorber mirror aremounted has an maximum edge dimension no larger than 350 mm.

(38) The method of any one of (29) through (37), wherein pumping thegain medium comprises providing between 2 Watts and 10 Watts of opticalpower to the gain medium and an average output power from themode-locked laser module is between 350 milliwatts and 3.5 watts.

(39) The method of any one of (29) through (38), wherein afull-width-half-maximum pulse width of the optical pulses is between 9picoseconds and 38 picoseconds.

IV. Conclusion

Having thus described several aspects of several embodiments of amode-locked laser, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art. Such alterations, modifications, and improvements are intendedto be part of this disclosure, and are intended to be within the spiritand scope of the invention. While the present teachings have beendescribed in conjunction with various embodiments and examples, it isnot intended that the present teachings be limited to such embodimentsor examples. On the contrary, the present teachings encompass variousalternatives, modifications, and equivalents, as will be appreciated bythose of skill in the art.

For example, embodiments may be modified to include more or feweroptical components in a laser cavity than described above. Moreover,laser cavity configurations may differ from those shown with some lasercavities have more or fewer turns or folds in the optical path.

While various inventive embodiments have been described and illustrated,those of ordinary skill in the art will readily envision a variety ofother means and/or structures for performing the function and/orobtaining the results and/or one or more of the advantages described,and each of such variations and/or modifications is deemed to be withinthe scope of the inventive embodiments described. More generally, thoseskilled in the art will readily appreciate that all parameters,dimensions, materials, and configurations described are meant to beexamples and that the actual parameters, dimensions, materials, and/orconfigurations will depend upon the specific application or applicationsfor which the inventive teachings is/are used. Those skilled in the artwill recognize, or be able to ascertain using no more than routineexperimentation, many equivalents to the specific inventive embodimentsdescribed. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, inventiveembodiments may be practiced otherwise than as specifically describedand claimed. Inventive embodiments of the present disclosure may bedirected to each individual feature, system, system upgrade, and/ormethod described. In addition, any combination of two or more suchfeatures, systems, and/or methods, if such features, systems, systemupgrade, and/or methods are not mutually inconsistent, is includedwithin the inventive scope of the present disclosure.

Further, though some advantages of the present invention may beindicated, it should be appreciated that not every embodiment of theinvention will include every described advantage. Some embodiments maynot implement any features described as advantageous. Accordingly, theforegoing description and drawings are by way of example only.

All literature and similar material cited in this application,including, but not limited to, patents, patent applications, articles,books, treatises, and web pages, regardless of the format of suchliterature and similar materials, are expressly incorporated byreference in their entirety. In the event that one or more of theincorporated literature and similar materials differs from orcontradicts this application, including but not limited to definedterms, term usage, described techniques, or the like, this applicationcontrols.

The section headings used are for organizational purposes only and arenot to be construed as limiting the subject matter described in any way.

Also, the technology described may be embodied as a method, of which atleast one example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used, should be understood to controlover dictionary definitions, definitions in documents incorporated byreference, and/or ordinary meanings of the defined terms.

Numerical values and ranges may be described in the specification andclaims as approximate or exact values or ranges. For example, in somecases the terms “about,” “approximately,” and “substantially” may beused in reference to a value. Such references are intended to encompassthe referenced value as well as plus and minus reasonable variations ofthe value. For example, a phrase “between about 10 and about 20” isintended to mean “between exactly 10 and exactly 20” in someembodiments, as well as “between 10±δ1 and 20±δ2” in some embodiments.The amount of variation δ1, δ2 for a value may be less than 5% of thevalue in some embodiments, less than 10% of the value in someembodiments, and yet less than 20% of the value in some embodiments. Inembodiments where a large range of values is given, e.g., a rangeincluding two or more orders of magnitude, the amount of variation δ1,δ2 for a value could be as high as 50%. For example, if an operablerange extends from 2 to 200, “approximately 80” may encompass valuesbetween 40 and 120 and the range may be as large as between 1 and 300.When exact values are intended, the term “exactly” is used, e.g.,“between exactly 2 and exactly 200.”

The term “adjacent” may refer to two elements arranged within closeproximity to one another (e.g., within a distance that is less thanabout one-fifth of a transverse or vertical dimension of a larger of thetwo elements). In some cases there may be intervening structures orlayers between adjacent elements. In some cases adjacent elements may beimmediately adjacent to one another with no intervening structures orelements.

The indefinite articles “a” and “an,” as used in the specification andin the claims, unless clearly indicated to the contrary, should beunderstood to mean “at least one.”

The phrase “and/or,” as used in the specification and in the claims,should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used shall only be interpreted as indicating exclusive alternatives(i.e. “one or the other but not both”) when preceded by terms ofexclusivity, such as “either,” “one of,” “only one of,” or “exactly oneof.” “Consisting essentially of,” when used in the claims, shall haveits ordinary meaning as used in the field of patent law.

As used in the specification and in the claims, the phrase “at leastone,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

The claims should not be read as limited to the described order orelements unless stated to that effect. It should be understood thatvarious changes in form and detail may be made by one of ordinary skillin the art without departing from the spirit and scope of the appendedclaims. All embodiments that come within the spirit and scope of thefollowing claims and equivalents thereto are claimed.

1. A mode-locked laser module comprising: a base chassis; a mode-lockedlaser having a laser cavity assembled on the base chassis; a gain mediumlocated in the laser cavity that exhibits a positive thermal lensingvalue between one diopter and 15 diopters when the mode-locked laser isproducing optical pulses; a pump source arranged to provide a pump beamthat is absorbed by the gain medium and causes the positive thermallensing; and a diagonal rib extending diagonally across the chassis thatincreases torsional stiffness of the chassis, wherein an intracavitybeam of the laser cavity passes through multiple openings in thediagonal rib.
 2. The mode-locked laser module of claim 1, wherein thepump source comprises a laser diode arranged to excite the gain mediumwith the pump beam.
 3. The mode-locked laser module of claim 1, whereinthe gain medium comprises a solid state crystal that is disposed in amount and has no active cooling.
 4. The mode-locked laser module ofclaim 2, wherein the mode-locked laser produces optical pulses stablywithout mechanical adjustments to the laser cavity for thermal lensingvalues varied over a range from 8 diopters to 12 diopters due to changesin optical power of the pump beam.
 5. The mode-locked laser module ofclaim 2, wherein the mode-locked laser produces optical pulses stablyfor thermal lensing values varied over a range from one diopter to 15diopters due to changes in optical power of the pump beam.
 6. Themode-locked laser module of claim 5, wherein the changes in the opticalpower of the pump beam are between 2 Watts and 10 Watts and an averageoutput optical power from the mode-locked laser module is between 350milliwatts and 3.5 watts.
 7. The mode-locked laser module of claim 1,wherein a pulse repetition rate of the optical pulses is between 50 MHzand 200 MHz and a maximum edge length of the base chassis is not morethan 350 mm.
 8. The mode-locked laser module of claim 1, wherein a pulserepetition rate of the optical pulses is between 50 MHz and 200 MHz andwherein the module has a slab form with a maximum edge length measuringnot more than 350 mm and a thickness measuring not more than 40 mm and aweight of the module is no more than 2 kilograms.
 9. The mode-lockedlaser module of claim 1, wherein a pulse repetition rate of the opticalpulses is between 50 MHz and 200 MHz and wherein a maximum volumeoccupied by the mode-locked laser module is not more than 0.1 cubicfeet.
 10. The mode-locked laser module of claim 1, wherein afull-width-half-maximum pulse width of the optical pulses is between 9picoseconds and 38 picoseconds.
 11. The mode-locked laser module ofclaim 1, wherein the gain medium comprises neodymium vanadate(Nd³⁺:YVO₄).
 12. (canceled)
 13. The mode-locked laser module of claim 1,further comprising a saturable absorber mirror mounted on a plate at anend of the laser cavity, wherein the plate is configured to be adjustedwith only two degrees of freedom which do not include angle adjustmentswith respect to an optical axis of an intracavity beam of the lasercavity that is incident on the saturable absorber mirror.
 14. Themode-locked laser module of claim 13, wherein the plate comprises aprinted circuit board having a metal coating or the plate comprises aplate of metal.
 15. The mode-locked laser module of claim 13, wherein afirst beam waist of the intracavity beam within the gain medium isbetween 100 microns and 150 microns and a second beam waist of theintracavity beam at the saturable absorber is between 75 microns and 125microns.
 16. The mode-locked laser module of claim 13, furthercomprising: a first focusing optic located within the laser cavity; anda laser window or optical flat located within the laser cavity, whereinthe first focusing optic and laser window or optical flat are arrangedto adjust an incident angle of the intracavity beam on the saturableabsorber mirror by rotating the laser window or optical flat.
 17. Themode-locked laser module of claim 13, further comprising a cavity lengthextending region located within the laser cavity between the gain mediumand the saturable absorber mirror, wherein the cavity length extendingregion folds the intracavity beam at least four times.
 18. Themode-locked laser module of claim 17, wherein the cavity lengthextending region comprises: a first reflector; and a second focusingreflector located between the saturable absorber mirror and the gainmedium, wherein the first reflector and the second focusing reflectorfold the intracavity beam three times on successive reflections.
 19. Themode-locked laser module of claim 17, wherein the cavity lengthextending region comprises a first reflector that folds the intracavitybeam multiple times.
 20. The mode-locked laser module of claim 1,further comprising: an output coupler located at a first end of thelaser cavity; a saturable absorber mirror located at a second end of thelaser cavity; a first focusing optic located within the laser cavitybetween the gain medium and the saturable absorber mirror; and a secondfocusing optic located within the laser cavity between the firstfocusing optic and the saturable absorber mirror.
 21. The mode-lockedlaser module of claim 20, wherein an intracavity beam between the firstfocusing optic and the second focusing optic is essentially collimated.22. The mode-locked laser module of claim 20, wherein a focal length ofthe first focusing optic is between 240 mm and 260 mm and a focal lengthof the second focusing optic is between 240 mm and 260 mm.
 23. Themode-locked laser module of claim 22, wherein the output coupler islocated between 280 mm and 300 mm from the first focusing optic and thegain medium is located between 4 mm and 8 mm from the output coupler.24. The mode-locked laser module of claim 1, further comprising only onemirror located within the laser cavity that provides angular adjustmentof the one mirror while the mode-locked laser is operating.
 25. Themode-locked laser module of claim 1, further comprising afrequency-doubling crystal mounted on the chassis and arranged to doublea frequency of an output beam from the laser cavity.
 26. A mode-lockedlaser module comprising: a base chassis; a mode-locked laser configuredto produce optical pulses and having a laser cavity assembled on thebase chassis; an output coupler mounted on a first mount at a first endof the laser cavity, wherein the first mount provides no angularadjustment of the output coupler with respect to an optical axis of anintracavity beam that is incident on the output coupler; a saturableabsorber mirror mounted on a second mount at a second end of the lasercavity, wherein the second mount provides no angular adjustment of thesaturable absorber mirror with respect to the optical axis of theintracavity beam that is incident on the saturable absorber mirror; anda gain medium located on the optical axis between the saturable absorbermirror and the output coupler, wherein a first beam waist of theintracavity beam within the gain medium is between 100 microns and 150microns and a second beam waist of the intracavity beam at the saturableabsorber is between 75 microns and 125 microns. 27-39. (canceled) 40.The mode-locked laser of claim 26, further comprising a pump sourcearranged to provide a pump beam that is absorbed by the gain medium,wherein the gain medium exhibits a positive thermal lensing valuebetween one diopter and 15 diopters when the mode-locked laser isproducing optical pulses.
 41. The mode-locked laser of claim 26, whereinthe gain medium comprises a solid state crystal that is disposed in amount and has no active cooling.
 42. The mode-locked laser of claim 26,wherein a pulse repetition rate of the optical pulses is between 50 MHzand 200 MHz and a maximum edge length of the base chassis is not morethan 350 mm.
 43. The mode-locked laser of claim 26, wherein a pulserepetition rate of the optical pulses is between 50 MHz and 200 MHz andwherein a maximum volume occupied by the mode-locked laser module is notmore than 0.1 cubic feet.
 44. The mode-locked laser of claim 26, whereinthe gain medium comprises neodymium vanadate (Nd³⁺:YVO₄).
 45. Themode-locked laser of claim 26, further comprising a diagonal ribextending diagonally across the base chassis that increases torsionalstiffness of the base chassis, wherein an intracavity beam of the lasercavity passes through multiple openings in the diagonal rib.
 46. Themode-locked laser of claim 26, wherein the saturable absorber mirror ismounted on a plate at an end of the laser cavity, and wherein the plateis configured to be adjusted with only two degrees of freedom which donot include angle adjustments with respect to the optical axis of theintracavity beam that is incident on the saturable absorber mirror. 47.(canceled)
 48. The mode-locked laser of claim 26, further comprising: afirst focusing optic located within the laser cavity; and a laser windowor optical flat located within the laser cavity, wherein the firstfocusing optic and laser window or optical flat are arranged to adjustan incident angle of the intracavity beam on the saturable absorbermirror by rotating the laser window or optical flat.
 49. The mode-lockedlaser of claim 26, further comprising a cavity length extending regionlocated within the laser cavity between the gain medium and thesaturable absorber mirror, wherein the cavity length extending regionfolds the intracavity beam at least four times.
 50. The mode-lockedlaser of claim 26, further comprising: a first focusing optic locatedwithin the laser cavity between the gain medium and the saturableabsorber mirror; and a second focusing optic located within the lasercavity between the first focusing optic and the saturable absorbermirror.
 51. A method of operating a mode-locked laser, the methodcomprising: providing a pump beam to a gain medium of the mode-lockedlaser, wherein the gain medium absorbs the pump beam and exhibits apositive thermal lensing value between one diopter and 15 diopters whenthe mode-locked laser is producing optical pulses; and producing theoptical pulses stably without mechanical adjustments to a laser cavityof the mode-locked laser for thermal lensing values varied over a rangefrom 8 diopters to 12 diopters due to changes in optical power of thepump beam.
 52. The method of claim 51, further comprising providing thepump beam from a laser diode.
 53. (canceled)
 54. The method of claim 51,further comprising producing the optical pulses stably for thermallensing values varied over a range from one diopter to 15 diopters dueto changes in optical power of the pump beam.
 55. The method of claim51, further comprising producing the optical pulses at a pulserepetition rate between 50 MHz and 200 MHz, wherein a maximum edgelength of a base chassis on which the mode-locked laser is assembled isnot more than 350 mm.
 56. The method of claim 55, further comprisingdoubling a frequency of an output beam from the laser cavity with afrequency-doubling crystal mounted on the base chassis.
 57. The methodof claim 51, further comprising producing the optical pulses at a pulserepetition rate between 50 MHz and 200 MHz, wherein a maximum volumeoccupied by the mode-locked laser module is not more than 0.1 cubicfeet.
 58. The method of claim 51, wherein a full-width-half-maximumpulse width of the optical pulses is between 9 picoseconds and 38picoseconds.
 59. The method of claim 51, further comprising folding theintracavity beam at least four times in a cavity length extending regionlocated within the laser cavity between the gain medium and thesaturable absorber mirror.
 60. The method of claim 59, wherein thecavity length extending region comprises a first reflector that foldsthe intracavity beam multiple times.
 61. A mode-locked laser modulecomprising: a base chassis; a mode-locked laser having a laser cavityassembled on the base chassis; a gain medium located in the laser cavitythat exhibits a positive thermal lensing value between one diopter and15 diopters when the mode-locked laser is producing optical pulses; apump source arranged to provide a pump beam that is absorbed by the gainmedium and causes the positive thermal lensing; and a frequency-doublingcrystal mounted on the chassis and arranged to double a frequency of anoutput beam from the laser cavity.
 62. The mode-locked laser module ofclaim 61, wherein the mode-locked laser produces optical pulses stablywithout mechanical adjustments to the laser cavity for thermal lensingvalues varied over a range from 8 diopters to 12 diopters due to changesin optical power of the pump beam.
 63. The mode-locked laser module ofclaim 61, wherein a pulse repetition rate of the optical pulses isbetween 50 MHz and 200 MHz and wherein the module has a slab form with amaximum edge length measuring not more than 350 mm and a thicknessmeasuring not more than 40 mm and a weight of the module is no more than2 kilograms.
 64. The mode-locked laser module of claim 61, furthercomprising: a saturable absorber mirror mounted in the laser cavity; anda cavity length extending region located within the laser cavity betweenthe gain medium and the saturable absorber mirror, wherein the cavitylength extending region folds the intracavity beam at least four times.65. A mode-locked laser module comprising: a base chassis; a mode-lockedlaser configured to produce optical pulses and having a laser cavityassembled on the base chassis; an output coupler mounted on a firstmount at a first end of the laser cavity, wherein the first mountprovides no angular adjustment of the output coupler with respect to anoptical axis of an intracavity beam that is incident on the outputcoupler; a saturable absorber mirror mounted on a second mount at asecond end of the laser cavity, wherein the second mount provides noangular adjustment of the saturable absorber mirror with respect to theoptical axis of the intracavity beam that is incident on the saturableabsorber mirror; a gain medium located on the optical axis between thesaturable absorber mirror and the output coupler; a first focusing opticlocated within the laser cavity; and a laser window or optical flatlocated within the laser cavity, wherein the first focusing optic andlaser window or optical flat are arranged to adjust an incident angle ofthe intracavity beam on the saturable absorber mirror by rotating thelaser window or optical flat.
 66. The mode-locked laser module of claim65, wherein a pulse repetition rate of the optical pulses is between 50MHz and 200 MHz and wherein a maximum volume occupied by the mode-lockedlaser module is not more than 0.1 cubic feet.
 67. The mode-locked lasermodule of claim 65, wherein the saturable absorber mirror is mounted ona plate in the laser cavity, and wherein the plate is configured to beadjusted with only two degrees of freedom which do not include angleadjustments with respect to the optical axis of the intracavity beamthat is incident on the saturable absorber mirror.
 68. The mode-lockedlaser module of claim 65, further comprising: a first focusing opticlocated within the laser cavity between the gain medium and thesaturable absorber mirror; and a second focusing optic located withinthe laser cavity between the first focusing optic and the saturableabsorber mirror.
 69. A method of operating a mode-locked laser, themethod comprising: providing a pump beam to a gain medium of themode-locked laser, wherein the gain medium absorbs the pump beam andexhibits a positive thermal lensing value between one diopter and 15diopters when the mode-locked laser is producing optical pulses; andfolding the intracavity beam at least four times in a cavity lengthextending region located within the laser cavity between the gain mediumand the saturable absorber mirror.
 70. The method of claim 69, furthercomprising producing the optical pulses stably without mechanicaladjustments to a laser cavity of the mode-locked laser for thermallensing values varied over a range from 8 diopters to 12 diopters due tochanges in optical power of the pump beam.
 71. The method of claim 69,further comprising producing the optical pulses at a pulse repetitionrate between 50 MHz and 200 MHz, wherein a maximum edge length of a basechassis on which the mode-locked laser is assembled is not more than 350mm.
 72. The method of claim 69, wherein the cavity length extendingregion comprises a first reflector that folds the intracavity beammultiple times.